There are many ways to do this. Here is how I do it:

The title bar on the main screen of this app is "Policy Profiler 32"

In top of the app's main.prg:

With _Screen
   .visible=.F.
ENDWITH
	
IF CheckWin('Policy Profiler 32')
   MESSAGEBOX('Policy Profiler is already running.'+CHR(13);
	 +'Please check your taskbar for the app'+CHR(13);
	 +'and click on its button to maximize it.' ,48,'Policy Profiler is already running!')
   RETURN .F.
ENDIF

With _Screen
   .visible=.T.
ENDWITH

*-------------------------------------------
FUNCTION CheckWin
PARAMETER tcTitle

* Routine which sees if an application is active by looking for its
* Window title
* expC1 Window title

DECLARE INTEGER FindWindow in Win32API as FindWindow STRING, STRING

RETURN IIF(FindWindow(0,tcTitle)>0,.T.,.F.)

>How can I prevent users from starting multiple instances of my application exe?