Do your users really understand what they are asking with this request? The filter "Bills to Pay" does not show these Bills - so if users leverage that filter (recommended) then they won't be paying Bills by mistake. There may be cases where, even though Odoo shows you should not show the Bill, you do in fact need to pay it. Blocking isn't always better than training users to use the filter and explaining that they are not blocked for the cases that break the rule.

Hi, Ray thanks for answering, you are right, if I define a real process such elements should not happen. The truth is that in real scenarios, users sometimes make mistakes, and it would be nice to mitigate such when possible.

I should be at least show a warning message, based on that field, that calls the attention of the payer to go further (research) and avoid errors committed by repetition.