ERP guides
What your ERP catches, and what it lets through
QuickBooks Online
QuickBooks Online has a preference under Account and Settings > Advanced > Other preferences that warns when a bill number already used for that vendor is entered again. It warns; it does not stop the bill being saved.
Xero
Xero flags possible duplicate bills and credit notes when the Contact, Reference and Total all match. You review them from a notification or the Review duplicate bills option and choose to keep, void or delete. It does not block saving, and bills created from the same repeating template are not compared.
Oracle NetSuite
The Duplicate Number Warnings accounting preference (Setup > Accounting > Preferences > Accounting Preferences) can be set to No Warnings, Warn (UI only) or Warn and Block. Warn and Block covers vendor bills and vendor credits, among other transactions, and compares a number with other records of the same type.
Microsoft Dynamics 365 Business Central
Turning on Ext. Doc. No. Mandatory in Purchases & Payables Setup means a purchase invoice or credit memo cannot be posted without the vendor's invoice or credit memo number.
Sage Intacct
Accounts Payable configuration has a setting for bills, checks or adjustments assigned duplicate numbers, with the choices Show warning, Do not allow transactions to be created, or Do nothing. Bills and adjustments are checked per vendor.
Why the built-in check is not enough
Duplicate-bill checks compare the invoice number, and sometimes the vendor and amount, as they were typed. The duplicates that get paid look different to the system: INV-1043 keyed once as 1043, the same supplier set up under two vendor records, a statement or reminder paid as well as the invoice, or a payment run sent twice. Finding them means normalising invoice numbers and comparing on vendor, amount and date windows across the paid history, then asking each supplier for the money back.