Fix cases where member presence can be stomped on

disco/state.py

@@ -184,6 +184,7 @@ class State(object):
         self.channels.update(event.guild.channels)
 
         for member in six.itervalues(event.guild.members):
+            if member.user.id not in self.users:
                 self.users[member.user.id] = member.user
 
         for presence in event.presences:
@@ -285,6 +286,7 @@ class State(object):
         for member in event.members:
             member.guild_id = guild.id
             guild.members[member.id] = member
+            if member.id not in self.users:
                 self.users[member.id] = member.user
 
     def on_guild_role_create(self, event):