Let's imagine you have an error instead of a warning. What will be your action at a scenario when the same supplier each fiscal year start with invoice no 1.
Year 2020 Invoice no 1 and Year 2021 Invoice No 1.
Now you will face a problem, isn't so?

Not really. In some countries the invoice number will never be the same for the same supplier. 

Then I guess, this is a development for your customer, you should ask developer to create such control .

I would add - before you add such control, think twice - I come on a need to create two invoice records for one invoice to handle specific cases. 

Hello, you can check this.

www.greytrix.com/.../