You need to provide more information.
Does this happen at all networks or just 1?
if you use the /whois command on the nick, does it show an address that looks similar to the address showing for the nick you are using? or is it someone else who chose to use that same nick?
/whois mags1
There's a chance it could be you somehow launching 2 copies of mirc and one of them connects without you noticing, but it is hard to know from what you have said so far.
