Do you have that in a alias? In that case msg will call msg and send it again. Where did you put it? If you put it to remote (ALT + R) in mirc, then you need to add alias to the line.
alias msg msg $$1 04,01 $+ $$2-
but it's not a good idea to change a built in feture, maybe bether to do.
alias cmsg msg $$1 04,01 $+ $$2-
In this line you call a cmsg.