How to prevent user to open multiple instance of an application?

If you were creating a Windows application, most often you would be doing long task like database creation, validation or some complex operation before form showing to user. so the patient user might wait few seconds to let the form show, others might launch the application again. To avoid user launch multiple instances of the same application, select the "Make single instance application" check box to prevent users from running multiple instances of your application as shown in below figure. The default setting for this check box is cleared (unchecked) so it allows multiple instances of the application to be run.

Steps to follow:

Right click on project --> Click on Properties --> Select Application Tab --> Check Make single instance application.