Quick update: I spoke to two helpful Apple Specialists last Monday. First we tried replacing Music Library from a Time Machine backup about a month ago, but it made no difference. We finally had to turn off Sync, trash my old library and turn Sync back on. At that point all the duplicate songs were gone (I went from 25,000+ to 12,000+, but the duplicate playlists remained. I saved all my songs on an external drive, but it was easiest (but time consuming) to download fresh copies to the iMac. That's completed successfully.
Next, I deleted all the duplicate playlists. The five smart playlists with no names, stayed deleted, but several other Smart playlists (60s, 70s, 80s, Last Added, etc) won't stay deleted. All of my personal playlists that were duplicated and deleted, have stayed gone, at least for now. I'm just leaving the duplicate Smart playlists for now. Maybe I'll try again after I update to Big Sur 11.2.
I'm still very curious if anyone besides Wayne and myself are seeing this problem. Thanks.
Jim