I'm still impressed with this tool - Thanks Microsoft.

It struck me that SyncToy has all the information available to provide
something as a bonus: a list of files in the left window that are duplicated
in the left window by name, size, date etc. In other words, files that you
have two copies of, in the same source destination, but don't necessarily
want duplicated there.

I know that tools exist elsewhere that identify duplicate files on a hard
disk but I suspect that reducing unwanted/unknown duplication of files would
be a common desire for most SyncToy users. And I recognise that's not quite
what SyncToy is intended to do but it would help people to keep their Sync
activities efficient (not Sync-ing duplicates) and their left and right
windows logically tidy.