This question got closed as it was an exact duplicate of the question asked by the same user just a moment ago. And rightly it got closed. But the link to the original question (which used to appear when questions were closed as duplicates) is missing. This will lead to close-reopen wars. Is this a bug?
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Looks like it was kind of an edit conflict. After the question was posted it got closed almost immediately. While it got closed the OP edited the post again (added a ";"). When saving this change he was still inside of the 5-minute grace window and the new change silently overwrote the automatically inserted duplicate link.
So I would think that the automatic "edits" by Community are stored in the DB differently enough from normal edits that they break the edit grace period algorithm.