Photos with identical image numbers

I was wondering how iPhoto handles images that have the same image number. My camera is about to hit 10.000 and from what I've been told the image number will start over at 00001. Will iPhoto append the image number so 2 photos don't have the same image number or will it just leave both photos named 00001?

Also, if I ever decide to drag a large group of photos to another drive will my computer tell me certain files already exist (see in that they have the same file name)? My library is getting so large I may want to take a large batch of photos and move them to a backup drive.