Well, you could do this by simply setting a %var and increase it whenever you start mIRC, but this way is a little bit cooler, IMHO wink

The principle behind this is that you start and name a DDE Server whenever you open up mIRC. If you then already are running an instance of mIRC, it will shut down the newly opened program.

I just made it simple and it may be malfuntioning every now and then, but hopefully it will give you an idea of how to do it

Your'n

Code:
  
on *:START:{ 
  inc %i 
  DDESERVER ON $+(mIRC,%i) 
  [color:green];We start the DDE Server and name it mIRC2 for example[/color] 
  if ($dde($+(mIRC,$calc(%i - 1)),connected)) {
    [color:green];if $dde(mIRC1,connected) returns something, there is another instance of mIRC running. [/color] 
    [color:green];You could use $ddename instead, but this would give people an idea of how to use $dde.[/color] 
    dde mirc1 command "" /ECHO -ag * The other instance of mIRC has been shut down
    [color:green];Let us echo to our first instance that we shut down the newly opened mIRC[/color] 
    exit
    [color:green];And here we exit that instance.[/color] 
  }
}
on *:EXIT:dec %i
[color:green];And then we need to decrease %i whenever we exit mIRC[/color]

Propogater @ DALnet, #HelpDesk and #Help