[Sprint/Zero](bug): Duplicate message bug fix

Closing #380 (closed)

Firstly I've mapped the this.messages array over to an array containing just the GUIDs, then I check whether the length of the GUID array is greater than 0, and there is more than one message incoming.

If that is the case then I filter out all entries in response.messages that have a matching entry inside of the keys array. (As they are duplicated).

Spent a while testing this, as it's an important one. I've tested between different accounts and in different situations (e.g. already have a large amount of messages, first time messaging etc).