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On my own system I've modified the testing code as follows

    if s1 != s2:
        return False

    if shallow and s1 == s2:
        return True

Which works as I expected it to work

If attributes aren't identical - Always fails
If the attributes are identical and it's in shallow mode - returns 
true
If the attributes are identical and it's not in shallow mode - 
goes on to check if the files are byte identical.

Whether there should be additional modes for finding byte 
identical files with different names, attributes etc. is another 
matter.