A duplicate invoice is rarely anyone being careless. It's usually a process where the same document can enter twice through two different doors, and nothing at either door is checking.
Worth understanding how they arrive, because the prevention follows directly from the cause.
How duplicates actually get in
Chasing. You haven't paid, the supplier chases, and re-sends the invoice. The chased copy gets entered because it looks new. This is the most common route, and note the irony — the duplicate is created by the delay, so slow processing causes the problem it then has to catch.
Two channels. The supplier's system emails an invoice and their account manager also sends a PDF. Both arrive, at different addresses, sometimes days apart.
Statement entered as invoices. A statement lists invoices you may already have. Enter it line by line and you've duplicated everything on it.
Re-issued with a new number. The supplier corrects something trivial and issues a fresh reference. Now the two copies don't share an invoice number, and the obvious check misses it.
Forwarding. Someone forwards an invoice to accounts, and someone else forwards the same one, and both get entered because each looks like a new email.
Part-payment confusion. An invoice paid in two instalments gets re-entered for the balance rather than the payment being allocated.
The checks that catch them
Layered, because no single check catches all of the above.
Same supplier, same invoice number. The obvious one, and it should be a hard block rather than a warning. Catches straightforward re-sends.
Same supplier, same amount, close dates. Catches re-issues with a new reference, which the first check misses. This one has to be a warning rather than a block — legitimate identical invoices exist, particularly for recurring services.
Same amount, same date, different supplier records. Catches the case where a supplier exists twice in your system under slightly different names. That underlying problem is worth fixing separately.
Total against an existing unpaid balance. Catches part-payment confusion.
Any accounting package worth using does the first check. The second is where most of the remaining value is, and it's often the one that isn't switched on.
Prevention beats detection
Three structural changes that remove the causes rather than catching the results.
One address for everything. Removes the two-channel route entirely, and it's the same change that stops invoices getting lost. Suppliers send to one place; nothing arrives by two doors.
Process faster. Duplicates created by chasing disappear when there's nothing to chase. This is the least obvious prevention and possibly the most effective.
Never enter a statement. Use it to reconcile — which is what it's for — rather than as a source of entries. Worth being explicit about with anyone new, because statements look enough like invoices to be entered in good faith.
And one supporting habit: keep the supplier list clean. Duplicate supplier records defeat every check above, because the system sees two different suppliers. One record per supplier, with variant names as aliases rather than as separate accounts.
When one has already been paid
It happens. The recovery is straightforward if you move reasonably quickly.
1. Confirm it's genuinely a duplicate. Two invoices with the same number and amount usually is. Two with the same amount and different numbers might be two legitimate invoices for identical recurring work — check what each relates to before claiming an overpayment.
2. Tell the supplier. Most will offset it against your next invoice, which is the simplest resolution for both sides. Ask for a credit note so there's a document trail.
3. If there's no next invoice, request a refund. Reasonable for a one-off supplier or a large sum, and you're entitled to it.
4. Correct the ledger properly. The duplicate bill needs reversing or crediting, not deleting — deleting removes the audit trail and makes the bank reconciliation harder to follow later.
5. Check the bank reconciliation. An overpayment that's been reconciled needs the correction to flow through, or your reconciliation stops agreeing.
Suppliers are generally reasonable about this. It's a common enough occurrence that most have a process, and raising it promptly makes it administrative rather than awkward.
Finding ones you haven't noticed
Two checks, neither of which takes long.
Reconcile supplier statements. The routine safety net. A statement shows what the supplier thinks you owe; if their figure is lower than yours, a duplicate is one of the few explanations.
Sort payments by amount for the last year and look for exact repeats to the same supplier. Recurring amounts will be legitimate; a one-off that appears twice is worth opening.
Worth doing once a year even in a process you trust. Most businesses that check find at least one.
A note on the numbers
You'll see large figures quoted for what duplicate payments cost businesses annually. Treat them cautiously — they're usually drawn from large-enterprise studies and don't transfer to a business processing fifty invoices a week.
The honest framing for a smaller business is that duplicates are infrequent and individually expensive, and the checks that prevent them cost almost nothing to switch on. That's a good enough reason without needing a dramatic statistic.
Cribble flags likely duplicates before they reach approval — matching on supplier and reference, and on amount and date where a re-issue has changed the number. Statements are recognised as statements rather than read as invoices.