A user already present in a chat joins the chat again.

[inetic]

This bug is exposed by the test_session_join_order test. The pseudo algorithm that the test implements goes like this:

• user0 waits for user1 and user2 to enter the room.
• Once user1 and user2 are in the room, user0 invites them both
• When user1 receives the ConversationInterface::joined_chat event, the Conversation::participants() function returns [user0, user1, user2]
• When user2 receives the ConversationInterface::joined_chat event, the Conversation::participants() function returns [user0, user1, user2]
• Once everyone has joined everyone else's chat, a new user is created with name "new_guy" and is invited by user0 into the chat.
• While user0, user1 and user2 are waiting for the "new_guy" to join the chat, user1 (or sometimes user2) receives a ConversationInterface::user_joined_chat("user2") (or user2 receives user_joined_chat("user1")).

The expected behavior is that user1 does not receive user_joined_chat("user2") event as user2 was already in the conversation (as known from the third bullet from the top).