Question: Once in a while, some well-meaning user appends the string " [closed]" to a question title after receiving a good answer. This is, of course, confusing to people who, for example, visit the question later and don't see a "closed as *reason* by *voters* at *time*" box.

I can't think of any case where this would be a desired behavior, so I propose rejecting any title that ends in " [closed]" except for the ones generated by the system for actually-closed questions. A quick explanation under "Oops! Your question couldn't be submitted because:" should be enough to make this feature non-confusing.

Answer: 
You're right, there's no reason to ever allow this — added a check for it.

EDIT:  

The filter has been updated to check for the strings "[migrated]", "[on hold]", and "[duplicate]" as well.
How about a distinct style for questions that are closed? Perhaps a red title or something of that sort would do.