The problem was that I followed this procedure:
1. I clicked on the node that lists a given duplicate. Two tracks showed in the main window.
2. I selected the one I wanted to drop, and dragged it to the node of the other track.
3. Nothing happened.
I now realize I should have done this:
1. Expand so that both tracks of a duplicate show as separate nodes.
2. Click the node for the unwanted track. It will show in the main window.
3. Select it in the main window and drag it to the node of the other track.
4. It works!
Sorry, but I thought your instructions were a bit confusing...