This is a bigger problem than most people realize.
Once you make backups and do restores of data it can sometimes duplicate every photo in your gallery, and it can be a huge pain to get around. For iOS there are a couple of apps that specifically address this problem.
The best one that I have found is called Duplicate Photos Fixer. It has a bunch of extra features like grouping suspected duplicates together in a way that you can easily zoom and get rid of all but one of them. Here is the direct link:
I know that this is a 99 cent app, but it really is very refined.