Well, that's exactly what your code is telling it to do - each time you click any of the three buttons, it will add -another- instance of the same widget to the layout.
What do you really want to do? Add the widget once, the first time the button is clicked and never again? Destroy the old widget (if any) when the button is clicked and then add a new one? Or something else?
Write down (in words) what you want to happen, then try to write the code to do that. Report back here on youor results, but not before you try that.
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