Are you saying that you have duplicates of images in the "Originals" folder?! I hardly ever use iPhoto but I thought one of its advantages was that it
always kept originals and any and all edits completely seprate, as in the "Originals" folder and the "Modified" folder. Maybe this is a left over system from an earlier, less intelligent iPhoto version.

Exactly where are these duplicates?
Studied Responses:
1. Yes, exactly (In fact, I composed my plea, I thought, quite carefully in anticipation of a TeeEss inquisition!)
2. I thought so, too.
3. Or, as I suggested – perhaps from "migrations" of various sorts between differing Macs, since the appearance of the first iPhoto in OS X .
4. Precisely when identified, above . . . I'm now wondering how to deal with duplicate photos already resident in my iPhotoLibrary.
On the very first screen of PhotoLibrary, Event 2 and Event 3 "share" 9 (possibly) duplicate photos. That easy to fix: hose the second 9. It's the (likely) dozens of single repetitions distributed across the whole schmeer, i. e. from Event 1 through Event 264 that has me hoping to avoid kathy's accurately designated "pain(s)"
Curiously, it's having been "freed" from some of my age-related, vexing "inconveniences" of vision (via recent cataract extraction) and arthritic hands (via the new iMac's fantastic screen and it's priceless new small keyboard + its fantastic speed with SnowLeopard) that even makes this proposed clean-up seem feasible. You "youngsters" won't understand – really – but the seasoned citizens among this assembly will.
Have you, at any time, 'shared/synced' your library with your iPhone?
Yup. I have a "For iPhone" album in iPhoto. It is to that repository that I direct iTunes when syncing.
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 . . . .
Not likely, since my syncing is "automatic" and the number of duplicates is far less than the number of those "automatics" over the past 3 years!
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.
Your surmise is correct. Some photos are scanned prints from the 40s, if you can believe that. There are few if any "dates"in iPhotoLibrary that reflect anything but when down/uploaded. I have wondered frequently how that might be changed for the future, if not for the already collected/assembled past
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've previously made a half-hearted attempt to do that by establishing "subject-matter" albums and putting, oh perhaps, a couple of 1000 items in such cartegories, then by inspection (only) try to prune duplicates. But that only affects the Albums not the Library, of course.
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.
I have enough trouble divining the fundamental "features" of my PSE 6 to worry about other bells-and-whistles I had never heard of ere this!
My present plan is to try Paddy's suggestion of This little puppy
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my syncing is "automatic"
I never allow iTunes to do anything automatically!
And if you mean you are also using MobileMe, you are flirting with danger of other content 'spawning' duplicates.
Finding duplicates of very large 'items' is not a trivial task for a computer. It cannot 'see' much of anything except the data. I see that that software looks at the data in a computer way. Possibly adding the values of bits in each row and column and coming up with a total that will very likely match only an extremely limited number of different images. Thus, any matches could easily be assumed to be duplicates, which could then be displayed for you to make the final decision of what to delete. Having a fast computer is essential, since you have thousands of images to compare. Even then, it could easily take some hours for the final display to appear. I can imagine ways that would eliminate most of the obvious non-duplicates and that could cut the search and the computations to mere minutes, but it might also miss some because the filtering criteria is not precise enough. At any rate, 8 bucks seems a very reasonable cost and well worth saving the time and frustration of doing this task manually! After all, that's why we have computers!