For the Spam Quarantine, users aren't notified if their post was sent there. As a moderator, you can remove legit posts from the quarantine, so they reappear on the community. It's best practice to check the quarantine on a regular basis to avoid legit posts from staying in there too long. Duplicate posts can be handled by removing one of the duplicates (don't mark as spam unless it's actual spam) to a private board and letting the remaining post live on the community. You could PM the user to explain that you removed their duplicates but left the original on the community, so they are aware.
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