This question already has an answer here:
Minor key and its chords
My guess is that message appears when a question is closed as a duplicate. I'm pretty sure that's where I've seen it.
This question is an exact duplicate of:
Inaccurate reputation graphs - bug or as designed?
I'm not sure about this one, sorry. I'm not sure I've even seen it...
This question may already have an answer here:
Is the message "This question may already have an answer here" shown after flags or only after close votes?
I think that's the message shown when OP views their own question, probably as motivation to check out that question and make sure it's not a duplication. I think that message appears when a duplicate is suggested by another user (but not yet closed or marked as a duplicate).
Are modes and scales the same?
I remember seeing those exact words as comments under the post when a duplicate is suggested, but I can't recall ever seeing a UI for it.
Pretty sure 1) is correct, but the others slipped my mind.