When I imported my photos from iPhoto to Aperture huge numbers of duplicates somehow were generated. I spent many hours deleting them, trying not to delete the masters, etc.
It was horrible, duplicates being spawned like in some bizarre video game where I wasn't good enough to shoot them all. This was probably the worst experience I ever had with software.
I have no idea what happened except that programs were developed to deal with it, so it was a widespread problem.
BTW, thanks Apple for that. What a huge pain bug to unleash on people. Somebody evidently wanted that program sent out prematurely. I can only guess who did things like that...
Got Duplicate Annihilator and it automatically got rid of the the dups, thus saving me much time. The problem is that the first version I got also deleted some masters evidently and I spent a lot of time dumpster diving in Aperture trash to retrieve photos I wanted to keep. Now there is a newer version of the program that presumably works better.
So, that is my experience. Have fun.