Restrict users doing copy for previous invoice

Hi

we need to restrict employees entering invoice without enter PO Number in Java enter invoice form .

We have done some personlization on enter invoice form ,and it was worked successfully ,but the user when do enter invoice by doing copy (click on shift+f6 button) for previous invoice in entering invoice form ,

The personalization is not working .

Please advise how I can restrict users doing copy for previous invoice .

I will be grateful if someone update me soon.

Thank you in advance