Controlled Vocabulary

Which One of These Is Not Like the Others?

(or Using Checksums to Deduplicate Image Collections)

As your image collection grows you will find that your responsibilities expand to include not only things like the prevention of data loss, but elimination of waste and inefficiencies..and one of the biggest wastes of time and money are those caused by redundant files.

If you have duplicates in your collection, you incur inefficiencies in storage and backup (and the associated costs); but more importantly, these inefficiencies in your management of those files create a huge drain on your time. After all, if you do happen to run across a file with multiple copies, it's difficult to know which one you may have "touched" (worked on) most recently without additional investigation.

Those of you that have tightly controlled processes for adding files may find the following interesting, but not particularly useful. For the rest of us, if we took the time to review the files in our collection we would likely find a fair number of duplicates.For those who manage files for others in an office or workgroup, there are probably more duplicates than they even realize (or want to think about). The question is how to find these redundant files without it becoming an overly burdensome "finding a needle in a haystack" task.

Using Checksums for Data Deduplication

The method outlined in this article shows you how to evaluate checksums or hashes to "deduplicate" your collection. Data Deduplication involves scouring your hard drive / server, etc. for redundant instances of files and selectively deleting them. While the text which follows makes reference to image files, the same process can be used for any file type on a computer. If you are managing a large number of image files that should not be modified (like proprietary Raw files), then you will definitely find the following information beneficial.

As image files take up substantial space, being able to eliminate all but a single instance could significantly decrease your need for storage, especially if you haven't always had a strict file handling process when first adding files. If you have had a hard drive crash, it's quite common to end up with multiples of files when you reload all of the information, especially if a lot of that was on CDR or other removeable media. While hard drive storage is becoming cheaper all the time, if you envision moving all or part of your image collection to the "cloud" or other remote storage, it really doesn't make much sense to have multiple copies of the same image asset within the same collection.


You will need to have a utility installed which is capable of creating a Checksum file, such as those discussed in the "Trouble Transporting Tribbles: or File Verification using MD5 Checksums" article, a spreadsheet application like Microsoft Excel, and some time. The goal is to gather all checksums together for the image collection we want to deduplicate, sort them by their checksum values, and then use a spreadsheet calculation formula to help us quickly identify the duplicates -- if any exist.  If there are duplicates, then we can use the spreadsheet as our checklist for deleting the files.

In short we need to:

    1. Create a master checksum text file for the image collection
    2. Import that text file into a spreadsheet application
    3. Locate recurring checksum values within the spreadsheet
    4. Find corresponding digital files and verify they are the same
    5. Delete all but one instance of the file.

Explaining exactly how MD5 checksums are created is beyond the scope of this article. If you are interested, try reading RFC 1321 "The MD5 Message-Digest Algorithm" at  (

Types of Duplicates

There several types of duplicates that might exist for any image file on your hard drive. Dictionary definitions usually include both broad and narrow meanings, such as: "Something additional of the same kind" or "A copy that corresponds to an original exactly."

In terms of digital images there are several types of duplicate files with which we might be dealing:

Some may also refer to derivative files as "similars" though this definition is used a bit too loosely for my taste, as it is often employed to refer to any images taken during the same shooting session having the same or similar characteristics, such as subject, action, or appearance.

Deduplication only spots "Exact" Duplicates

I realize this might sound a bit redundant, but the deduplication method outlined will only allow you to match "exact" duplicates.  Due to how checksum hashes are calculated, only files that match bit-for-bit will have matching values. So, for instance, this process will not help you locate files where:

It will, however, allow you to find images that are the same even if the file modification date has changed, or the filename is different. It's not uncommon to have images that have been downloaded from a website be stamped with the date they were download as the file creation or modification date. In corporate workgroups it's very common to find the same file renamed by someone else using a file name that makes sense to them. Multiply by the number of users and you could easily have 4 or 5 versions of some popular files.

The great news about only finding exact duplicates is that once you have found a match (and verified them), your only decision is which file to keep; and even that decision may not be that difficult as often the choice to delete or retain will be based on where the file is located. As the two files are true "duplicates" you don't have to worry much about whether one image is newer, or has had it's metadata worked on, as this process will only flag them as duplicates if they are bit-for-bit the same, including metadata. However, rather than indiscriminately deleting files you might want to first look to see if there is any pattern to the duplicate files that you find. You may find that a particular folder full of files was copied with a different folder or file names.

If you are trying to locate images that visually match each other, but are not identical duplicates, there are other methods you might want to pursue (these will be addressed in a future article).

Creating the Master Checksum File

If you have not created checksum files for your folders or drives that has to be done first. If you are not familiar with that process, see the previous article, "The Trouble Transporting Tribbles: or File Verification using MD5 Checksums." At end of this article, I will share some sample data of my own to give you a rough idea of how much time this process might take. Once you have gathered together the text file(s) with those checksums and the file paths, then you can add the calculation formula to find the duplicates. Note, while the examples below use MD5 checksum values, the process will be the same if you are using other types of checksums such as SHA-1, or SHA-2. The only requirement is that all hash values be of the same type. In other words if you put both MD5 and SHA-1 checksum values into a spreadsheet to evaluate, the process can't locate duplicates.

What's in a Checksum file?

Checksum+ and MD5summer both use a plain text file containing one line for each file and its corresponding checksum in the format: CHECKSUM<whitespaces>FILENAME. The delimiter between the filename and checksum is always several blank spaces; tabs don't seem to be used. So, you'll have to either manually search and replace these whitespaces or make the necessary adjustments when importing the data into a spreadsheet for sorting. MD5Summer has a couple of additional lines/rows before the data, whereas Checksum+ just starts with the data.

Here is what an MD5 checksum file from MD5summer looks like for a single folder with no subfolders when opened in notepad.

Here is what an MD5 checksum file from Checksum+ looks like for a single folder with no subfolders when opened in a the Apple Textedit program.

Here is what an MD5 checksum file from MD5Summer looks like when the "Add Recursively" option is used to capture a folder with subfolders:

Here is what an MD5 checksum file from Checksum+ looks like for a folder with subfolders:

"Mash" Your Checksum Files

In order to compare the checksum values, you will eventually need to have all this information in a single file, and/or imported into a spreadsheet. If you ran a checksum operation on an entire drive or volume, then you may only need to import that single file. This makes for an easy import, but the creation of that checksum file might take a while.

If you already have a number of checksum files from various folders, you might find it takes less time to import each in turn and create a master checksum text spreadsheet (in other words create a "mashup" of all the separate files). If that is your plan, you'll probably find it easier to move a copy of each of those checksum text files to a single folder, as the Get External Data option (in Excel) will return to the same folder each time. Another option, is to first open each of the separate checksum files in a simple text editor and combine them all into a single one before importing.

The Windows version of Excel 2007, which uses the "ribbon bar" is shown above. Below is the older style version of Excel for Mac OS X.


As there are a number of blank spaces between the checksum values and the filename or path/filename, using the Data Import option within your spreadsheet program, may be the simplest -- if this is provided as an option. For example in Excel, you can look under the Data tab or menu item and then find the Get External Data >> Import Text File menu item.  Then locate the Checksum file where it was saved. Note, if you are using MD5summer the checksum file that is created will have the ".md5" extension; so you will need to change the "Files of Type" pull-down to "All Files (*.*)" in order to "see" the file in the folder. The Checksum+ files use a standard .txt file format, so they should show up when you access the folder where they are stored.

When you import the file, there are a couple of different options in the import "Wizard." You can use the "Delimited" option, and then click "Next" and then check the box labeled "space" to tell Excel to use blank spaces as the marker between the two values. The other  "Fixed Width" option can also be used to separate the first set of characters (the hash) from the file/path name.

Expert Tip: if you are working with a collection of files where the users (or you) have blank spaces in the file or folder names you will definitely want to use the "Fixed Width" option. The "Delimited" option will interpret each blank space in a file or folder name as a delimiter, and you'll end up with many more than two columns of data. Also note that if you are using a file naming structure that has leading zeros, you'll want to change the cell format from "General" to "Text" as otherwise a file like 09122501.nef could have the leading zero dropped and become "9122501.nef."

In any event, you'll need to get all of the checksums representing all of the folders you want to check in a single spreadsheet before running the formula to locate the duplicates.

Include "Path" references

If you are joining together multiple checksum files you might want to  include the "path" to each file as well, to make it easier to locate files later -- especially if your checksum files only have the file name. Why that is important should be obvious, if it isn't it will be when you get to the deletion phase.

If you are importing checksum files that each represent a single folder, then adding the path to that folder in the column to the right of the file name for that set of imported entries will make it possible to know where that particular file is located (plus it's a lot easier than trying to find a way to insert it into the column where the file name is located.  If you are wanting to compare files on various drives or storage volumes, then you probably will also want to include those portions of the path names as well. Whether you wish to make these full, syntactically correct paths is up to you. The key is that you are trying to make it easy to locate the files that are flagged as duplicates so they can be deleted.

Clean Up Your Sheet

While not required, you may want to remove the detritus from the imported checksum files (such as the first three lines from each MD5summer file) and maybe even label the column headers, especially if you keep the file for reference.

Identifying Duplicate Files in the Spreadsheet

After you have all of your checksum files in the spreadsheet, you can sort by the checksums in column "A."  In Excel, first select all the columns with data, and locate the Sort & Filter button (under the "Home" tab in the ribbon bar) and then choosing "Sort A to Z." Any checksum hashes that match will now be located next to each other. 

Of course, visually scanning Column A for duplicate entries is mind-numbingly boring and unless you possess extreme proofreading skills, or have some OCD tendencies it's unlikely you will find them all. However, this is just the kind of job that computers (and spreadsheets) can handle easily no matter how many rows of data.

After you have sorted the data by the value in the Checksum column, insert a new column between A & B. You can call it "Dupe?" if you like. Simply click on the B at the top of column B and then Right-click (Windows) or Control + Click (Mac) and choose "Insert" from the contact sensitive menu. A new blank column should now appear between the checksum values and the file/path. You can also locate the Insert Column command under the Home tab in the ribbon bar of newer versions of Excel.

If you used the import wizard to set the cell formatting property to "Text" earlier, be sure to first change the property for this new blank column to "General" or another type that allows for formulas.

Click on the B at the top of column B again, and then Right-click (Windows) or Control + Click (Mac) and choose "Format Cells" from the contact sensitive menu. In the resulting dialog click on the Number tab and set the value under Category to "General."

In the second row of column b (B2), you will need to insert the following formula:


In laymans terms, what this formula does is to have Excel note the value of the current data cell (indicated as $A$2), and then check to see if that same value is in a given range (in this formula 999) of data cells below it. If a match is found, the formula returns a value of TRUE (rather than FALSE). The IF statement at the beginning of the formula says that if a TRUE value is found to print "dupe" into column B beside it (if the result is FALSE it will not print anything). This process keeps repeating up to the cell value noted after the colon (indicated as $A$999 above).

If you have more than 999 rows, then you will need to adjust the second instance of $A so that it notes the last row in your spreadsheet (otherwise be sure to press the Enter/Return key after adjusting the formula). So if you have 3525 rows, the formula would need to read:


After making the necessary adjustment for the number of rows, you then need to copy this formula into the rest of the cells. Now if you are thinking that this means a lot of copying and pasting, relax and take a deep breath. This will only take a handful of keystrokes.

First click on the cell where the formula is located (in our example this is B2).  Then either use the shortcut keys to "Copy" the value of this cell, or use the Copy command under the Edit menu.

Now use your cursor to click into cell B3. Then use the scroll bar to move to the last row in your spreadsheet. While holding the Shift key down, click into the last cell (this will highlight all of the cells between B3 and BXXX). Now use the shortcut "Paste" keys, or locate "Paste" under the Edit menu to replicate the forumula into the rest of the cells. If you click into B4, you will see that the formula is very similar, but not the same.  The value just before the close parentheses will increment to match the row of the cell.

So in row 2 the formula reads: =IF(COUNTIF($A$2:$A$3525,A2)>1,"dupe","")
and in row 9 the formula reads: =IF(COUNTIF($A$2:$A$3525,A9)>1,"dupe","")

You can now quickly scan Column B of your spreadsheet and see if you have any exact duplicates, which will now have the term "dupe" next to them. Before moving to delete these files, here are a few known file issues that might trip you up.

Known Issues that Might Trip You Up

There are some files that are likely to show up as duplicates, even though they really are not. These are more likely to appear when moving files from one computer operating system to another. They are noted here for your convenience, along with a little explanation of their purpose. There isn't any good reason to retain these in your spreadsheet. Whether or not you wish to remove the actual files from your hard drive is something you may want to test before deciding.

Double Format files
On most Macs, a "double format" is used in the file system and moving a file to a different file system will cause the prefix "._" to appear for each file name (a period and underscore followed by the file name). If, for example, you are creating checksums on the Windows platform for images created or moved from a Mac platform, there will be a "._drp2105047d.jpg" in the list along with "drp2105047d.jpg" when you view the report (unless you have deleted the "doubles" prior to creating the checksum). Due to these "doubles" being file system proxies, the underlying data is the same in all, and thus the checksums will all have exactly the same value (even though the larger files they reference are not duplicates). As these files are mated to the originals, it is safe to ignore them when locating duplicates. If the files are now residing on a storage device that will only be accessed on Windows, you could remove them without causing any harm.

Desktop Services Store Files
On Macs the ".DS_Store" (aka the Desktop Services Store) is a hidden file created by Apple Inc.'s Mac OS X operating system to store custom attributes of a folder such as the position of icons or the choice of a background image. This file is created by the Mac OS Finder and is used to keep track of icon types and positions, folder view options,  and other information about that  folder. There is also a separate .DS_Store file that is created for each directory to store information about that directory. As a result you'll find them appearing all over your hard drive, in virtually every folder that you have visited with the OS X Finder.

This Windows system file is a cache of the thumbnail pictures in that directory. It speeds up the showing of thumbnails when you are viewing a folder in Thumbnail view. These files should not affect performance, with the exception that they take up disk space. Although this space is very small in most cases, if you have a lot of thumbs.db files scattered across your hard drive you may be able to save some space by removing them.

If you want to prevent the creation of this cache file and delete all the thumbs.db files that are on your hard drive, follow the steps below:

  1. Open My Computer
  2. Click on Tools
  3. Click on Folder Options
  4. Click on the View Tab
  5. Place a check in the option "Do not cache thumbnails"
  6. Click Ok
  7. Close My Computer

Verify Before Deletion

Now that you have run your deduplication formula you should be seeing the word "dupe" appear next to each entry where the checksum appears twice (or more). Even though the checksum hashes happen to match for two or more files, it's still a good idea to visually verify that the images are the same before deleting.  You may want to open them in an image editor, or locate them within your image catalog, or look at the thumbnail in the Finder or Explorer window.

If you read the previous post, "File Verification using MD5 Checksums" that covered how to create Checksums, you might recall that "Each check/sum hash should be unique (unlikely to match other hashes for similar images)."  This doesn't necessarily rule out that while "unlikely" it's not "impossible" that there could be two different files with the same checksum that are not duplicates.

Having the drive location and path will make it a bit easier to locate the files for visual inspection and deletion.

Flag Deletions with a Color of Your Choice

I typically apply a color shade to the matches as I delete that file. If you  have a very long spreadsheet, you might also want to add a column and simply add a number 1 to that cell.  This will allow you to do a sort at the end, and move all of the matched duplicates to the top of the sheet.

If these are images that are already in your image cataloging software, it may simply require typing in both file names as part of a search.  If you are using an image cataloging program then you may wish to use the "delete" features within that application, as then you won't wonder later why you have a thumbnail for a file that no longer exists. If you aren't already using an image database you might want to consider one of those mentioned on the Image Database and Cataloging Software page.  

Some Real World Results

Here are some real-world results from one test using the free MD5summer checksum utility.

MD5summer took 41 mins, 55 secs to scan and create MD5 checksums for 771 files located on fileserver connected via ethernet. This image collection (a set of folders within a folder) occupied 15.9 gb on disk.

I used the formula described above on the spreadsheet I created from the MD5 checksum report and found 55 instances of duplicate files. After visually confirming that these were the same files, I manually removed the files. I was able to reduce this portion of my server to 714 files which took up 15.3 gb on disc (I didn't delete all files in one sitting, so I can't tell you how long the deletion portion took).

For other related articles, check the What's New page, or stay tuned to posts on Twitter, or the ControlledVocabulary Forum] if you are interested.

Many thanks to Dan Dill, Dave Klee, Alberto Mateo, Said Nuseibeh and Tony Schutz for their assistance in proofing and testing the suggestions in this article.

Initially posted: August 3, 2010

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