Only 1 instance of my script

Hi guys

If I have a script named "test.exe" running, and while it's running the script "test.exe" will run a new instance, how can I make sure that the second instance waits for the existing instance to finish?

I guess that I should use _Singleton ? But I can only find examples that kills the second instance, instead of waiting for the existing instance, and then run.

Thanks!

David