signature for a file ?

[Paul Rubin]

You don't say what OS you're running.  Anyway if two files have the
same MD5 checksum, it's basically certain that the files are
identical.

So your basic strategy is:

   for each file in your system:
      c = md5 checksum of the file
      print c and the pathname leading to the file

Then take the output of the above and sort it so all the identical md5's
end up next to each other in the output.

Then find all the sets of files with identical md5's and print out their
paths so you can investigate further, get rid of one copy, etc.