In the buy and sell category you're allowed to bump your thread every 24h:
13. Bumps, "updates" are limited to once per 24 hours.
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21. Old bumps should be deleted. [2]
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So instead of reporting it as double post you can report it as "user doesn't delete old bump posts" or something like that with links to the old bumps and the posts should be removed:
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13. When reporting a user for bumping more than once within a 24 hour period. Make sure to include as much evidence as possible. Either link to the other bump or include a archive of the page.
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21. When reporting multiple bumps it's best to report the earliest bump and only that one, and include a comment in the report field that there's more in the thread.
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