From XEP-0045:

The service MAY rewrite the new occupant's roomnick (e.g., if roomnicks are locked down or based on some other policy). The service MUST allow the client to enter the room, modify the nick in accordance with the lockdown policy, and include a status code of "210" in the presence broadcast that it sends to the new occupant.

Example 22:

<presence
    from='coven@chat.shakespeare.lit/thirdwitch'
    id='n13mt3l'
    to='hag66@shakespeare.lit/pda'>
  <x xmlns='http://jabber.org/protocol/muc#user'>
    <item affiliation='member' role='participant'/>
    <status code='110'/>
    <status code='210'/>
  </x>
</presence>

Beagle does not respect this new nickname given by the service on join, resulting in self messages appearing as duplicates in the chat view: first, a message with the nickname beagles thinks is my nickname, then, the actual "echo" of my message with the proper nickname that the MUC service has given me.