Better deduplication of answers

[jpd236]

If two people submit answers to the same puzzle at the same time, they both get added as distinct answers.

I'm not sure how easy this is to resolve in the general case, particularly if there are more than 1 intended solution. But it feels like adding a solution would ideally be wrapped in a read-modify-write such that if the new solution would cause us to exceed the expected solution count of the puzzle, it replaces the latest one rather than being added on top of it.

[zarvox]

If two people submit the same guess to the same puzzle at different enough times that the client submitting guess 2 sees guess 1, I believe we prompt them to reconsider (and if they persist, still allow them to create the duplicate guess). I'm not sure if we have the same guardrail in the no-guess-queue form variant, though, so that might be something worth checking/implementing if we don't yet!

If two people submit the same answer to the same puzzle at truly the same time, we usually have the operator manually reject one of the submissions as a duplicate to match the expected answer count. If you wind up with a puzzle that has two correct answers, you can always go back to the guess history page and mark one of the "correct" submissions as "rejected" instead, which I think should remove the duplicate answer on the Puzzle object.