Have you, at any time, 'shared/synced' your library with your iPhone? Apparently, one can 'share' pictures
from within iPhoto
to an iPhone. I would think it would only be possible from with iTunes, myself, but I've never done it in any manner. The point is, after the iPhone has some of these images, it is possible to sync things
the other way around and end up placing the iPhone images back into iPhoto.
I know this can sometimes cause problems with phone numbers and contacts, but I thought those issues were caused by using MobileMe.
At any rate, that is a possible
cause. Don't do that any more!
As to finding duplicates, the only way I can see is to search by date. Keywords would be great, assuming you took the monumental effort of creating unique words for all images!
And used that same word when importing them the second/third/etc. time. I didn't think you had done that, either.
The problem with searching by date, of course, is not only that you have them spread over ~5 years, but undoubtedly over many months and those dates will probably relate to when they were placed in the program not the time/date they were actually taken.
The only other way I can logically(?) think of is to export all mages (or subgroups) to individual specifically named folders (if that's even possible). At that point, hopefully, either they will automatically sort themselves by name or date. But that will still, of course, require visually scanning the list(s) for matching pairs/triplets/etc.
I don't know if you ever bought Elements, but it installs Adobe Bridge CS3 or CS4, depending on whether you have PSE 6 or 8. Bridge has vastly greater search capabilities than iPhoto (which wasn't really designed for that kind of work, to begin with) and it can look at image collections just about anywhere on you hardware. But even that searching would assume that some bit of data is similar on all duplicates
and that you can figure out what that info is, fairly exactly.