non-blocking communications in MPI: order of messages

Question

1. What happens if a processors sends exactly the same message to the same destination with the same tag? When the receiver wants to read it, does it read the last one?
2. What if two different processors send the same message (same tag) to one processor? which one it receive and in what order?

3- How we can know how many pending messages from a specific processor are in the queue in order to receive all of them?