Regarding the question title:
Should we close Questions as duplicates of old, unanswered Questions?
It is not possible anymore to mark a question as a duplicate, without the older/original thread having an upvoted or accepted answer on it. See here.
2013-02-01: The "close as duplicate" process has been updated: the interface to search for duplicate ...
I think this is a question that you will be better trying to resolve via Meta SE because it is wider than GIS SE.
To see multiple other reports there type "An error occurred submitting the question" into its search bar.
Alternatively, can you try submitting a minimalist question, and if that works, then build it up until you can no longer save it.
I suspect that this came about by caching on the device used by @kahukowhai but that is not something that I can be certain of. It may also have been that the user read your original question earlier and only got around to commenting on it much later without re-reading.
I agree that the sequence/timing of that comment seems odd, but I don't think there ...