Are they actual duplicates or is this a raw and jpg issue?
If the latter, you can select the folder that has the images, go into Library -> Grid View, and on the top you can select Text (searching) and search for jpg. It should find ALL of the jpg images in that folder and then you can select all and delete.
If it's the former, than they should have the word "copy" in the name (or something like that). Same thing: search for the word "copy" and then delete them.
If there's nothing unique about these images, put on some nice music and have a pleasant afternoon.