I've written a little tool [1] that shows file duplicates between a
wikipedia and Commons, as well as internal duplicates. It runs of a
static list created from the toolserver databases; currently, German
and English are available. I will have to regenerate the data for
other wikipedias and for updates manually.
But for now, there's ~29.000 dupes between en.wp and Commons, as well
as ~8.500 between de.wp and Commons, so it might take you guys a while
;-)
A subset (default:25) images is selected randomly from the list, so
you might run into images that already have {{NowCommons}}.
Cheers,
Magnus
[1]
http://toolserver.org/~magnus/cgi-bin/duplicate_images_across.pl?lang=en&am…