Short answer: detect duplicate invoices with several rules, not one: same supplier and same invoice number after normalising the format, same supplier and amount within a few days, and the same invoice under two supplier records. Prevent them with one way in, one queue, clean supplier records and a check before every payment run. If one has already been paid, ask for a refund or credit and correct the VAT.
Why duplicates happen and what they cost is covered in the quiet cost of registering the same invoice twice. This post is the working checklist.
What counts as a duplicate?
The exact copy is the easy one. These are the ones that get through:
- Same number, different format. "KP-2026/0187", "KP 2026 0187" and "0187" are the same invoice.
- Reminder with the original attached. It arrives weeks later and looks like a new document.
- Statement booked as a bill. A supplier statement lists open invoices. Entered as a bill, it duplicates all of them.
- Pro forma, then the final invoice. Both get registered.
- Credit note confusion. A credit note shares the supplier, amount and sometimes the number of the invoice it corrects. It is not a duplicate, but booked as a positive amount it becomes one.
- Corrected invoice without a credit. The supplier reissues with a new number. Without a credit note cancelling the first, both look payable. Under the EU VAT Directive (Article 219), a document that amends and refers specifically and unambiguously to the original invoice is treated as an invoice, so ask for one.
- PDF and e-invoice for the same supply. The supplier sends a Peppol e-invoice and also emails a PDF. A Factur-X or ZUGFeRD file is different: it is one PDF with the XML embedded, so it is one invoice, not two.
- Two clients' copies. In a firm, an invoice addressed to client A ends up in client B's pile because the director runs both companies.
Detection rules
Software usually checks repeated invoice numbers only within a single supplier record, as the Washington State Auditor's Office points out, so duplicate supplier records let duplicates through. Layer the rules. The windows below are starting points to tune, not standards.
| Rule | Normalise first | Likely meaning | Action |
|---|---|---|---|
| Same supplier + same invoice number | Remove spaces, punctuation, prefixes and leading zeros; ignore case | Duplicate | Block |
| Identical file or identical content | Compare the document itself, not the file name | Exact copy | Block |
| Same supplier + same gross amount + invoice date within 7 days | Use gross amount; match supplier by VAT number | Probable duplicate or reissue | Hold for review |
| Same VAT number or IBAN on two supplier records | Strip country prefix and spaces from VAT numbers | Duplicate supplier record | Merge records, then re-run the checks |
| Same invoice number + same amount, different supplier | As above | Wrong supplier chosen, or wrong client | Review |
| Buyer name or VAT number is not this client | Compare against the client's details | Invoice belongs to another client | Move it |
| Same supplier + same amount every month | Check the service period on the invoice | Usually legitimate (rent, subscriptions) | Allow if periods differ |
| Document type is credit note, statement, reminder or pro forma | Read the document title | Not a new bill | Do not register as a bill |
On normalising numbers: some suppliers restart numbering each year, so 0042 in 2025 and 0042 in 2026 can both be genuine. EU invoices must carry a sequential number that uniquely identifies the invoice (Article 226), but a series can include the year, so keep the year or invoice date in the comparison.
A duplicate invoice example
Kappa Print Ltd sends invoice KP-2026/0187 on 3 October: net 1,965.00, VAT at 18% 353.70, total 2,318.70. It is booked from the email.
On 24 October a reminder arrives with the invoice attached. A colleague keys it in as invoice "0187", dated 3 October.
- Supplier + invoice number: normalised, that is "20260187" against "187". No match. It gets through.
- Supplier + amount + date window: Kappa Print, 2,318.70, same date. Flagged.
That is why one rule is not enough. In a duplicates view with filters and search, the reviewer sees both copies side by side, links the second to the first and discards it.
Prevention controls
One way in per client. Every supplier invoice goes to one email inbox. Ask suppliers to stop posting paper and to send one format, not two.
One queue. Every incoming document, from the inbox, a phone photo or the client's upload, lands in the same list before anything is booked. Two people working from two piles is how the same invoice gets entered twice.
Supplier records you can trust. One record per VAT number. Few people allowed to create or edit suppliers, which the Washington State Auditor's Office also recommends. Check new suppliers added each month against existing VAT numbers and bank details.
Approval that shows the flag. The approver should see "possible duplicate of KP-2026/0187" before saying yes, not after. Set up purchase approvals so flagged bills cannot skip review.
A check on every payment run. Before release, sort the run by supplier and amount. Two equal amounts to the same supplier in one run, or a bill already paid last week, should stop the run.
How to fix a duplicate that was already paid
- Confirm it. Find both payments on the bank statement and both bills in the ledger.
- Work out whose duplicate it is. If you entered one invoice twice, the second bill is your error: void or reverse it. If the supplier invoiced twice, they need to issue a credit note that refers to the original invoice.
- Ask the supplier for a refund, or for the amount to be offset against the next invoice. Put it in writing with both payment dates and amounts.
- Correct the VAT. If input VAT was claimed on both bills in a return already filed, correct it. Whether that goes in the next return or needs a separate correction depends on local rules and thresholds, so check the rules where the client is registered or ask your VAT adviser.
- Match the refund when it arrives, against the supplier's debit balance.
- Fix the cause. Which rule would have caught it? Add it.
A month-end duplicate check
- Review everything flagged as a possible duplicate and clear the list.
- Run aged payables by supplier and look for equal amounts close together.
- Look for suppliers with a debit balance: that is often a payment made twice.
- For the three to five largest suppliers, reconcile their statement against your payables.
- Check new supplier records against existing VAT numbers and bank details.
- Note anything found and the rule that would have caught it earlier.
Ten minutes per client, and the bank reconciliation stops surfacing surprises.
Where Invoreg fits
Invoreg compares each new document with the documents already in the workspace and with bills already in the accounting software. A match is flagged and linked to the original, and the duplicates view lists every flagged copy. Invoices can be forwarded to a mailbox, every client sits in one firm-level queue, and purchase approvals (up to three levels) keep a flagged bill from going through unreviewed. It does not fix your supplier records or your payment run for you: those controls are still yours.
Try it on your own invoices. The first 20 invoices are free.