OK, so now I feel stupid. I had many duplicates because I had used several photos in posts on Google+. I wasn't aware that it was saving them twice. Thanks for your help and for sticking with me while I figured out my stupidness.
I thought you had tried all of that already--are you trying again? The next step is to open Gallery again, and wait a few seconds for the database to repopulate with local photos. Usually, the Gallery should not show any more Picasa photos in this situation, after doing what you just did.