Run only one copy of program

Now if I write an application, e.g. abc.exe, the user could run more then
one copy of the program when they click it in the explorer many times. So how
could I do if I want the user can only run only one copy of that program, i.e. if
the program (abc.exe) already started and the user double click its icon in the
explorer, then the abc.exe will got focus (not a new process).

Thank!

[HeSaidJoe]

' See if there is already and instance.
    If App.PrevInstance Then
 ' Activate the previous instance
        AppActivate App.Title
        
' Terminate the new instance
        Unload Me
    End If

it work, thank HeSaidJoe!