Delete duplicates in one folder only.

It seems that it is not possible to locate where in your collections papers in the "duplicates" folder reside. So, that brings me to my question of how to delete duplicates in just one folder/collection.
I have lots of collections that i have built up over years. I am now doing a review and have a specific folder of the search results from databases (2000+ papers). I would like to remove duplicates from this folder only. I don't want to remove the previously saved papers in other collections, which will undoubtedly be the same as those in the review folder. I would like to keep ONE copy in the review folder and ONE copy in the relevant collection. This is important because when the review is finished i will delete the review folder but need all the other collections to still have all the papers that they started with.
I hope this makes sense. Basically, how to i delete duplicates just from the review folder and not the rest of the collections.
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