Ignore join accept message if already joined

[reissjason]

There could be an instance when a join accept message is received inadvertently, then a new address would be assigned and new keys computed.

[mluis1]

The behavior that you are observing shouldn't happen as each node should have a different AppKey (increases the network security) which implies that the MIC computation should fail.
The proposed correction is not enough because by just applying your patch the nodes will no more be able to rejoin a network (unless by resetting the node). The LoRaWAN specification allows network roaming which implies that the node must redo a Join procedure in such situations.

On the next version I'll try to integrate a fix for this situation.