When the only difference is the name booth files should have the same content and size.

1. About the content. We can compare two files by the command diff file-1 file-2. Also we can use this command for a test in this way:

diff -q file-1 file-2 > /dev/null && echo 'equal' || echo 'different'

2. About the size. We can find a file with certain size by the command (where 12672 is the file size in bytes):

find /path/to/search -type f -size 12672c -printf '%p\n'

Or we can use a range in this way (where 12600-12700 is the file size range in bytes):

find /path/to/search -type f -size -12700c -size +12600c -printf '%p\n'

Note that, by default the command find works recursively.

3. Combine the two methods (where file-1 is our pattern file):

find /path/to/search -type f -size -12700c -size +12600c -printf '%p\t' -exec sh -c 'diff -q file-1 "$1" > /dev/null && echo "equal" || echo "different"' sh {} \;

4. Example. Let's assume we have the following directory structure:

$ tree /tmp/test
/tmp/test
├── file-1   # this is the pattern file
├── file-2   # this is almost the same file but wit few additional characters
└── file-3   # this is exact copy of file-1

The result of the above command will be:

$ find /tmp/test -type f -size -12700c -size +12600c -printf '%p\t' -exec sh -c 'diff -q file-1 "$1" > /dev/null && echo "equal" || echo "different"' sh {} \; 
/tmp/test/file-2        different  # OK: here we have added few additional characters
/tmp/test/file-3        equal      # OK: this is exact copy of file-1
/tmp/test/file-1        equal      # OK: this is file-1 compared to its self

Or we can simplify the output by changing our command in this way:

$ find /tmp/test -type f -not -name "file-1" -size -12700c -size +12600c \
  -exec sh -c 'diff -q file-1 "$1" > /dev/null && printf "%s\tis equal\n" "$1"' sh {} \;
/tmp/test/file-3        is equal

Update from the comments. The following command finding for file with the same size as the file-1, and then the diff commas is involved with --brief and --report-identical-files options:

find /path -type f -not -name "file-1" -size $(stat -c%s file-1)c -exec diff -qs file-1 {} \;
Files file-1 and /tmp/test/file-3 are identical

We can compare md5sum of the files in this way:

• Get the md5sum of the pattern file:

  $ md5sum file-1
  d18b61a77779d69e095be5942f6be7a7  file-1
  

• Use it with our command:

  $ find /path -type f -not -name "file-1" -size $(stat -c%s file-1)c -exec sh -c 'echo "d18b61a77779d69e095be5942f6be7a7 $1" | md5sum -c -' sh {} \;
  /tmp/test/file-3: OK
  

• You can search for a particular string with grep -rl "string" (-r for recursive, finding the string in files, -l for showing the filename, not the string)

This might take a while, but it should be effective and reliable. It assumes you are using Bash. Replace file with the name of your renamed file:

shopt -s globstar
for i in **; do [ -f "$i" ] && cmp --silent file "$i" && echo "$i"; done

• shopt -s globstar turns on recursive globbing with **. You can turn it off with shopt -u globstar, but it is off by default and will be off when you open a new shell.
• for i in ** loop over all files below this one. Run the command from the highest level directory that might contain the file or the directory with the file or the directory... (apply recursion to this sentence!)
• [ -f "$i" ] && if the file is a regular file that exists then...
• cmp --silent file "$i" && if there is no difference between file and the file being examined (ie if cmp exits successfully), then...
• echo $i print the relative path of the file (this also prints the path of file itself, but I didn't see much benefit in fixing that).

Thanks to this answer on Stack Overflow for the cmp method of comparing files.

If you're looking for (or okay with) a GUI application, you may try the "FSlint Janitor" application. You can install it by running

sudo apt-get install fslint

How to use the application:

Once installed, follow the steps below.

1. Launch the application.
2. Select the "Duplicates" option (1) to search files with the same content.
3. Click on the "+ Add" button (2) and select the directories to look for files (make sure the "recurse" option is checked to include sub-directories).
4. Click on the "Find" button (3) and wait.

Grep can find it quickly

When used properly, the grep command can find the duplicate quickly. You must be careful not to search the whole file system or it will take days to complete. I recently documented this here: `grep`ing all files for a string takes a long time

For optimum speed use:

grep -rnw --exclude-dir={boot,dev,lib,media,mnt,proc,root,run,sys,/tmp,tmpfs,var} '/' -e 'String in file'

If your file might be on a Windows directory remove the mnt directory.

If you know the file is within the /home directory someplace you can shorten the command:

grep -rnw '/home' -e 'String in file'