As long as the user does not have a given chat open, the client should be free to reorder all unread incoming messages there according to timestamp.
Otherwise, when a message is received with a timestamp that is earlier than the most recent one on that timeline, clearly mark it in a special way. Possibly also provide an information bubble that explains that this can be caused be resending.
Possibly also provide a button next to it that would make the message jump to its place in chronological order. Or duplicate the message at both places: show it chronological as a normal message and also at the time of reception as a dim one as if it was a reply to the original one (i.e., so a jump-to button was shown).