How to Find Duplicate Payments and Invoices
Why duplicate payments happen, the exact and fuzzy tests that find them, how to tune the date window, and the controls that stop them recurring.
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Duplicate payments are an unglamorous, steady leak. Industry studies commonly put them at a fraction of a percent of payables — small per payment, large in a year. Recovering one is awkward (you have to ask a supplier to refund), so prevention and early detection matter.
How duplicates happen
- The same invoice entered twice, once from the emailed PDF and once from the statement.
- A supplier re-sends an invoice with a slightly different number (GS-1042 vs GS-01042).
- A corrected invoice is issued without cancelling the original.
- A supplier statement is paid on top of the invoices it summarises.
- The same bill paid by both cheque and bank transfer.
- Vendor master duplicates — one supplier set up twice under different spellings.
The tests
Work from simple to fuzzy:
- Exact: same supplier + same invoice number + same amount.
- Normalised reference: ignore case, spaces, hyphens and leading zeros so INV-0042 matches inv42.
- Same amount + same supplier within N days: catches re-keyed numbers.
- Same amount + different supplier: catches duplicate vendors. Expect more noise.
- Same amount + same date: often a double-click or a repeated batch.
Tuning the window
A narrow window with the supplier required gives few, strong flags — start there. Widen the window to 30–90 days to catch statements paid after invoices. Relax the supplier rule only if you suspect duplicate vendor records. Expect false positives from legitimate repeats: rent, subscriptions, instalments.
Do it in minutes
Export your purchase ledger or payment run as CSV and paste it into the free Duplicate Transaction Finder. It groups matches, shows the money at risk (every copy after the first) and exports the groups. Everything runs in your browser; nothing is uploaded.
What to do with a flag
- Agree both entries to the supplier invoices.
- Check the bank — was it actually paid twice?
- If paid twice, request a refund or a credit note and record it, rather than netting it silently against a later invoice.
- Record the root cause.
Controls that prevent it
- Unique constraint on supplier + invoice number in the system.
- Pay from invoices, not statements, or reconcile statements to the ledger first.
- Three-way match (order, goods received, invoice) for stock purchases.
- Vendor master hygiene: one record per supplier, change approvals, periodic clean-up.
- Reconcile the bank monthly — see bank reconciliation step by step.
Systems that hold purchases, payables and bank lines in one ledger make duplicates visible sooner. Pyalm Books covers bills, suppliers and purchase orders alongside banking and reconciliation, and is free during beta. If you'd rather test more broadly, add a Benford's Law test and a check for sequence gaps.
Standards, rates and deadlines change. Confirm the current position with the relevant authority or a qualified adviser before you rely on it.