[richardwhiuk]

The standard fix to this is to give each message a GUID, and then when you delete a message, instead of deleting it, your store a tombstone with the GUID, and that prevents a network re-send from causing the message to reappear.

[texasbigdata]

Sorry, what’s a tombstone? Like add a field that says “deleted”?

[Toutouxc]

Yes, it's a concept used e.g. in hash tables that use open addressing [0], where you can't delete value X because the final address of value Y may depend on whether X was present when it was being inserted. So instead of deleting X and leaving behind an empty address we change X to a tombstone that stays there forever and says "something was here and if you're looking for Y, Z or anything else, keep on looking".

[0] https://en.wikipedia.org/wiki/Hash_table#Open_addressing

[texasbigdata]

Thank you

[intricatedetail]

Or store message encrypted and corresponding key, then just delete the key.