Use App.PrevInstance to see if a previous instance is already running.

Private Sub Form_Load()
    ' See if there is already and instance.
    If App.PrevInstance Then
        ' Activate the previous instance
        AppActivate App.Title

        ' Send a key (here SHIFT-key) to set the
        ' form from the previous instance to the
        ' top of the screen.
        SendKeys "+", True

        ' Terminate the new instance
        Unload Me
    End If
End Sub