Comment 48 for bug 136702

While knowing size and last modified are useful things to know,
they are not necessarily enough to tell that two files are identical
(in particular as sizes are usually only approximate).

Better would be to have the system find out for me that
the two files are identical by computing a checksum (md5 or similar)
in the background and let me know the result: "Note: the files
are identical", or "Note: the files are not identical". This is
very quick on all but the largest files (you may want to ask
specifically for *.iso and similar).