Duplicate messages when remove recieved messages is off

When removing received sms messages is set to off via /mm-sms rm_r off purple-mm-sms duplicates messages that have already been received.

This seems to happen when purple-mm-sms polls ModemManager for messages (I believe this happens when it starts up). I'm guessing purple-mm-sms gets a full list of messages and doesn't check for duplicates of messages it already knows about.

I think this may be related to chatty#287, but it's not quite the same bug.