https://www.youtube.com/watch?v=URifVfYlD18

https://www.youtube.com/watch?v=URifVfYlD18
In this article you'll learn how to "run-once enable" 
a Delphi application that can check for its previous 
(running) instance. Along the process, several techniques 
of implementing such a check will be discussed; as well 
as how to bring your already running application to the 
foreground, if a user tries to run it "one more time". By 
the end of the article you'll have a copy-to-go code to 
control the behavior of your application's multiple instances: 
with the option to limit the number of running instances
https://www.youtube.com/watch?v=URifVfYlD18