PDF overwriting previous PDF

Hello fellow Kioskeans,

When I generate an invoice from my invoice template, it is automatically saved to an "Invoice" folder as a pdf.
However, I have noticed that if the invoice has exactly the same client name (I have a number of repeat customers), the newest invoice overwrites the previous one in the same name once saved as a pdf. I have tried to create a Rename File message box giving me the option of renaming the newest invoice file from, say, "aaa.pdf" to "aaax.pdf" prior to saving it.
I tried to create a message box with the question: "Do you want to rename "aaa.pdf" to "aaax.pdf"? and the notification "There already is a file with the same name in this location" with a YES/NO option button.
However, I have not succeeded with the message box and I'm running out of hair!!

Does anyone have any ideas?

Thanks in advance.
Cheers,
vcoolio.