How to delete images already imported into iPhoto?
If you batch drag them, as jmlumpkin suggested, a number of them will be spotted with the 'do you want to import again', and you can apply no to all, then delete the bunch. If hwoever, it accepts them, there are a few bits of duplicate finder software - none of them free (at least not that I've found), but at around $8 - http://www.brattoo.com/propaganda/. 

I'd love to find something free that did the same!

Other helpful things: It may take some time, but letting faces do its thing, you will be able to group up and spot some duplicates.