I think you missed my point.

I wasn't talking about two masters sending identical messages at the same time. But two masters at each end of a long cable, where the two masters manages to correctly loopback their own transmission wnd don't notice that the other end of the cable is busy with a another transmission.

If transmitting to a single receiver, you will have to wait for the nack (should be avoided) or a missing ack.

If sending a broadcast, the master will not know if any receiver did pick up the message, unless the receivers are time-slotted to send staggered acknowledges.