Duplicate payments: how they happen and how to catch them automatically

How the same invoice gets paid twice

• The supplier emails the invoice and posts a copy; both are keyed.
• An invoice number is entered with a leading zero one time and without it the next.
• A payment fails, is re-run manually, then the original clears.
• A credit note is issued but the original payment is never cancelled.
• Two entities in a group settle the same intercompany charge.

Matching logic that works

Exact matching on supplier, amount and invoice number finds the easy cases and misses most of the real ones. A practical engine layers tests:

1. Same supplier, same amount, within a short date window.
2. Normalised invoice reference: strip spaces, leading zeros, punctuation and case.
3. Fuzzy supplier matching, so the same vendor spelled three ways still groups.
4. Same amount to different bank details for one supplier, which is the fraud case rather than the error case.

Keep the noise down

Recurring identical charges, such as rent or a subscription, will trip a naive rule every month. Exclude known recurring patterns and set a value floor so the report stays readable.

Recovering the money

Most duplicate payments are recoverable if raised within the supplier statement cycle. Reconcile supplier statements monthly, keep a duplicate register, and offset against future invoices rather than waiting on a refund.

Preventing the next one

Enforce unique invoice references per supplier, block payment runs against unmatched receipts, require callback verification for bank-detail changes, and reconcile the payables control account every period.

NanoFora flags likely duplicates with the matched entries and links to the source documents. See accounts payable process and controls.