Sort and remove duplicate photos
There are many options – and at least three different approaches to this:


Use a third party application to find (and remove) duplicate files. Probably the most convenient option – there is a variety of different apps like Duplicate Detective available – but you'll have to buy a license. There used to be a free demo version of Gemini, if you just want to find duplicates and delete them yourself. Haven't tried it for years though. 
Use the search function of your finder. This is a bit clumsy, but good method to find duplicates occasionally. You can search for "*" (asterix/wildcard) and then narrow your results down by using the filters and the sort options – e.g. "only images, modified after 1st of January 2014" or the like.
Use Terminal Commands or Apple Script. You should probably only go this direction if you know what you're doing.


There is a blog post "How to Find and Delete Duplicate Files in Mac OS X" that goes deeper into these three approaches.