A bill number once entered, cannot be repeated in Zoho Books except the following two cases:
Case 1: You have multiple vendors and a few of them might be following a similar series. Let’s say Vendor A and B have their bill number series as Bill-1, Bill-2, and so on. There are chances where you might have received bills with the same number from them. In such cases, Zoho Books lets you enter a bill number twice if they are from two different vendors.
Case 2: Your vendor repeats his bill number series every financial year. So, if you happen to receive a bill this year, you will be able to record another bill with the same number, from the same vendor in the next financial year only.
and GST filing.