If they are VERY closely synchronized, and there is at least a third node operating on the net with a different address, the message will be ACK'ed as correct by the third node and will appear as a single message. If the 2 nodes are not closely synchronized (some fraction of a bit width of time) then there will be a detectable error on the network, and one or both nodes will backoff and retry transmission according to their configurations. If there are ANY differences in the data at all, that will of course also cause a detectable collision/error event. Otherwise, the message is for all intent just a single message on the network.