I just voted to close a question which was equivalent to Goldbach's conjecture. When I had to give the reason, I didn't really know what to pick.
Off-topic? It is definitely a research question, so...
Duplicate? Well, hopefully not, at least not exactly.
When it was closed some hours later, I found out that three of the other voters had checked "off-topic - See Math.StackExchange to ask general questions in mathematics." Which is IMHO again not the best way to deal with it.
May I suggest to add another pre-defined possible reason for "off-topic"? Something like
"This question is identical or very close to a notorious open problem."