I must have blundered into doing something right. As a newbie I porting a group from Yahoo, I created a like-named group here. After several attempts to complete the port the way I wanted to, I finally got a good transfer. But I wanted the message numbers in the new group to match the Yahoo message numbers so inter-message references worked (e.g., see Message #553).
So I renamed my group with a "Test-" prefix and immediately afterwards created a new group with the original name. There were no issues creating the second group. I was going to delete the Test- group but two new moderators wanted to use it to experiment with the calendar so it remains active for now.