I don't know how to persuade 'find' to identify files in directory A that are identical to files in directory B. But my point stands - I can accomplish this quickly on the command line, and know of no GUI tools that would let me do it.

See. Your discussion proves my point perfectly. Too much hassle.

I can't work out if you're trolling or not, but your comment doesn't make sense at all. I needed to perform a task, and I did so fairly trivially from the command line. Someone else offered what they thought might be a shorter alternative, I pointed out that the task was more complex than they thought.

You take this as evidence that it can be done from a GUI?

So, let's go back to your comment. You said:

    I still haven't found a tool which
    doesn't have an equivalent GUI.

If you want to make me take you seriously, please tell me how you would:

    Given two directories,
        find all duplicate files that are
            both less than 10 minutes old

In other words, given directories A and B, find all filenames f such that:

    $A/$f is less than 10 minutes old
    $B/$f is less than 10 minutes old
    $A/$f is identical to $B/$f

Now tell me how you would do that every hour for a week.