Why duplicates slip through
Most AP systems block an exact repeat of the same vendor and invoice number. Duplicates that get paid are the ones that look different to the system but are the same bill to a person.
- The invoice number keyed two ways: INV-1043, 1043, 0001043 or 1043A.
- The same supplier set up twice in the vendor master, often after a remit-to address change.
- A statement or reminder paid alongside the original invoice.
- One invoice paid by check and again by ACH or card when a payment run is re-sent.
- An invoice paid by two entities in a group that share the supplier.
Matching rules that work
Normalise invoice numbers before comparing: strip spaces, punctuation and leading zeros, and treat a trailing letter as a possible re-key. Then compare on supplier, amount and a date window rather than on the raw invoice field.
- Same normalised invoice number and amount, different payment: strongest signal.
- Same supplier, same amount, invoice dates within 30 days, different invoice numbers: review.
- Same amount and invoice number across two vendor IDs with similar names or the same bank details: likely vendor-master duplicate.
- Credit memos already applied: exclude them, or the same dollars get claimed twice.
Recovering the money
Ask the supplier for a statement of account first. A credit sitting on their ledger is the fastest recovery: request a refund or apply it to the next invoice. Send the supplier both payment references, dates and amounts in one letter, and log the reply date so the follow-up is not lost.
Two to three years of history is usually the sweet spot. Older items are harder to evidence and suppliers may have written the credit off.
Stopping the next one
- Merge duplicate vendor records and block new vendors that share bank details with an existing one.
- Validate invoice numbers on entry with the same normalisation used above.
- Run the duplicate check before each payment run, not once a year.
Updated November 5, 2026