When a post with media takes the provider more than about 45 seconds, its gateway answers an error even though the post may still be published behind it. That answer used to count as a plain failure and the post was retried, which is how duplicates appear. Now a cut like that is treated as "maybe live": nothing is resent automatically, the system looks the post up at the provider and adopts it when it exists, and a person retries only after checking the network.