There was a bug for this, which was supposed to be fixed in an earlier TB 78. Does it work if you hold Ctrl while dragging and dropping?

No, holding Ctrl while dragging and dropping does not work, still duplicates entry in source.

What is the result in safe mode (hold Shift when launching TB)?

No, drag & drop does not work in safe mode, with or without the Ctrl key. Also, I just noticed that all 13 of my email lists also exist in my "All Address Books" and my "Collected Address Books" I never noticed that before so not sure if it's new. Let me explain a little more of my observations: I am president of an HOA so I have multiple email lists to support various categories: Owners, Directors, Non-Resident Owners, Renters, Residents, and also a list for each of our eight buildings. Names can exist in multiple lists and my name is in every list as an assurance that the email actually transmitted (my ISP has intermittent problems supporting VPN, not relevant here). Until this issue, I could add a new contact into All Address Books or Personal Address Book, or Collected Addresses and then drag & drop from there to as many email lists as appropriate. Last week I added a new renter into Personal Address Book and could not d&d to Renters, Residents, or to the correct building list, they belong in these three lists. Today I deleted the new contact completely and then tried adding the new contact into the Renters list, when I hit Enter, the new entry did not appear in Renters but was transported up to both Collected Addresses and All Address Books. So if I added the new entry into the three appropriate lists, I would wind up with three copies in the All Address Books and three more in the Collected Address Books and still none in the correct email lists. I hope this helps.

You might have corrupted address books, which can usually be repaired, but I would first suggest a simple test: create a new address book from File/New..., add a couple of test contacts, then select the new book and create a new list from File/New..., name it and close it, then add contacts from the new book to the list by drag and drop onto the list icon. Don't add contacts from All Address Books, as it is not a real address book, but just a search result.

This worked, I can d&d from the new address book to the new list. Surprise, surprise, this also seems to have corrected the original problem which somewhat confuses me. I have deleted the test List, test Contacts, and test Book. Everything now works as it should. I guess this series of tests just woke the system back up, or do I have a somewhat corrupted address books?

You'll know that your address books are corrupted if they don't retain additions or changes between TB restarts.