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'System & API - How to determine whether there's another (compiled) instance of the application running in the system
'The PrevInstance property lets you determine whether there's another 
'(compiled) instance of the application running in the system. This can 
'be useful if you want to prevent the user from accidentally running two 
'instances of your program:

Private Sub Form_Load()
    If App.PrevInstance Then
        ' Another instance of this application is running.
        Dim saveCaption As String
        saveCaption = Caption
        ' Modify this form's caption so that it isn't traced by
        ' the AppActivate command.
        Caption = Caption & Space$(5)
        On Error Resume Next
        AppActivate saveCaption
        ' Restore the Caption, in case AppActivate failed.
        Caption = saveCaption
        If Err = 0 Then Unload Me
    End If
End Sub