currently: (in telepathy-python/telepathy/server/channel.py:269)

    @dbus.service.signal(CHANNEL_TYPE_TEXT, signature='uuuuus')
    def Received(self, id, timestamp, sender, type, flags, text):
        self._pending_messages[id] = (timestamp, sender, type, flags, text)

This will always emit a signal

I suggest either:
creating a standard wrapper (so that users don't have to):

    def received(self, id, timestamp, sender, type, flags, text):
        """Wrapper around broken Received signal"""
        if id not in self._pending_messages:
            self.Received(id, timestamp, sender, type, flags, text)

or raising ValueError, to stop the signal being transmitted:

    @dbus.service.signal(CHANNEL_TYPE_TEXT, signature='uuuuus')
    def Received(self, id, timestamp, sender, type, flags, text):
        if id in self._pending_messages:
            raise ValueError("You can't receive the same message twice.")
        else:
            self._pending_messages[id] = (timestamp, sender, type, flags, text)