If you have a batch of messages for a given room from /sync beginning at a certain prev_batch, then obtain an earlier token by requesting a previous batch using /messages?dir=b, and then request events from the resulting batch token back to the prev_batch from /sync using /messages?dir=f, you should receive the same set of events, assuming nothing weird got added to the event graph in the intervening time. Instead, the final event in the second set of events consistently duplicates the first event in the sync response.

(Imported from https://matrix.org/jira/browse/SYN-775)

(Reported by Benjamin Saunders)