This question is not about error messages and duplicate invoice check in general, it is a very specific case about checking the fiscal year only.
Even an example is given.
If you check the invoice date, then it is not a duplicate since the date is different.
if you do not check on the date, then SAP checks the other conditions and may find invoices in other fiscal periods.
You wont change this by message customizing.
Please try to understand the question. And try the solutions in your system prior to posting as it may waste the time of the author.