Duplicate Invoice Numbers

Hello,

We upgraded from 11.5.9 to 12.1.3 recently.  I would like to confirm the criteria that will prevent a user from entering a duplicate invoice number.  Is it true that if you enter the same supplier name/number and invoice number it will provide you with an error that it is a duplicate invoice?  Does the site come into play with this criteria? If the supplier name/number/invoice number are the same but the sites are different will it be considered a duplicate?   If someone could provide me with a list of criteria that needs to be met in order to prompt the system