Update to above: I moved the old threads back into the wiki, and ran the URL manager. It found the two duplicate articles, and has allowed me to correct one of them.
I still have one document which displays as a page conflict, but clicking the Edit button gives me this error:
There is no page conflict for the specified URL.
Just to document what I'm going to do (which I suspect will fix it):
- Change the forum back to a normal non-wiki forum
- Move the unwanted duplicate thread out
- Change the forum back to a wiki again
- If that doesn't fix it, I will change it back to a non-wiki forum again, and edit the name of the unwanted thread, and then make it a wiki again, to see if it will let me properly remove the article.