These lines of code will allow you to exit a script if it is already running. It works best when the script is compiled into an EXE. If it's run as an uncompiled AU3 script it will quit if *any* AutoIt script is already running.
Hope this helps somebody.
#include <Process.au3>; Quit with message box if app is already running$numberOfAppInstances = ProcessList(_ProcessGetName(@AutoItPID))$numberOfAppInstances = $numberOfAppInstances[0][0]If $numberOfAppInstances > 1 Then MsgBox(0, "Exiting", "Application already running!") ExitEndIf