% cd Picturesin the Terminal. Be sure not to have other folders with "Originals" folders in them in your "Pictures" folder.
% rm -r */*/*/*/Originals
[Editor's note: This is a somewhat dangerous "rm" command if you're not comfortable in the Terminal, given the subdirectories it is traversing. If you'd rather remove the duplicates using the Finder, here's how. Just launch Find (Command-F), click on "Specific Places" in the "Search in" pop-up, and then drag your iPhoto archive (top level folder) into the listing of drives and make sure it's the only one selected. Then search for items whose file name contains "Originals". You'll get one window containing all the Originals folders; just drag them from that window to the trash, and you're done.
As an aside, of course, you will no longer be able to revert edited photos after doing this!]

