Flag compare should return more than just a boolean

I think an enum could potentially allow for better messages. For example if we returned something like

FlagStatus.DUPLICATE

A plugin could say something like you've already submitted this exact flag

Currently we just say correct/incorrect. I don't think this behavior necessarily needs to change but the ability to change it in a plugin would be nice.

No file, test, or entry point is identified in the issue. Start by locating the flag comparison behavior and its plugin interface, then determine how a status such as DUPLICATE could be exposed without changing the current correct/incorrect behavior; done means plugins can produce more specific messages.

Written by the indexing model from the issue text.