If Axolotl fails to decrypt a message, raising a DuplicateMessageException, a dummy message should be created
Consider the following scenario (it's something I actually experienced):
- A message is received, either directly or from MAM. It's decrypted but for some reason not stored or displayed to the user
- A MAM query occurs, the client receives a stanza corresponding to the missing message. Axolotl won't be able to decrypt it because it already has been.
Expected behavior: a placeholder is created in place of the missing message, indicating that it's there but cannot be decrypted Actual behavior: the message is silently dropped (or only mentioned in the logs)