I'm receiving duplicates messages on my mailslot! What is wrong?

Nothing, what you are seeing is the expected behavior. The reason for this is that you will receive a copy of a message for every protocol the sender and the receiver have in common. So, for example, if both systems have TCP/IP and NetBEUI installed, you'll get the same message twice.

To get around it, add a unique ID to each message you send. Of course, the ID only has to be unique for your program, so you can use, for example, a counter. On the receiving end, simply discard messages with ID's you've already seen.