I went to edit out the in-question duplicate. But pressing the edit button shows the error:
Suggested edit queue is full
The post is locked, as well as merged. When I attempt to edit a non-merged locked question, the much better error text is given:
Post is locked.
I have edit privileges on SO, so I don't believe the Suggested Edit Queue should be involved in this process at all. So I'm assuming it's a bad error message, not the fact that the queue is actually full or that my edit would be queued.