Here another approach to check if a script was already started using atoms and semaphores.   Atom: #include <MsgBoxConstants.au3> Global $iSingleton = Singleton() If Not $iSingleton Then Exit MsgBox($MB_TOPMOST, "Singleton Test", "Process is already running!") EndIf MsgBox($MB_TOPMOST, "Singleton Test", "Singleton atom initialized: " & $iSingleton) Singleton_Delete($iSingleton) ; #FUNCTION# =======================