To catch duplicate invoices before you pay twice, check every new invoice against what you have already entered or paid. The same supplier and the same invoice number is an exact duplicate: stop it. The same supplier and the same amount within a short window is a possible duplicate: hold it for a person to look at. Regular same-amount charges, like rent, get a note instead of a block.
Why do duplicate invoices happen?
Almost never because a supplier is trying to trick you. Duplicates usually come from ordinary paperwork habits on both sides.
- Sent twice. The supplier emails the invoice and also posts a paper copy. Or they send a reminder with the same invoice attached, and it gets entered again.
- A statement treated as an invoice. A vendor statement lists open invoices and a balance. If someone enters the balance as a new bill, every invoice on it gets paid a second time.
- A corrected invoice with a new number. The supplier fixes a price and reissues the invoice under a new number, but the original is still open in your system.
- A typo in the invoice number. INV-1043, INV1043 and 1043 are the same invoice to a person, but three different invoices to a system that compares text exactly. A letter O typed for a zero does the same.
- The vendor is in the master file twice. "ABC Supply Inc." and "ABC Supply" have different vendor IDs, so a check that only looks within one vendor never sees the match.
The last two are the reason a simple "invoice number already exists" rule is not enough on its own.
What is the difference between an exact and a possible duplicate?
It helps to treat these as two separate checks with two separate responses.
| Type | How it matches | What to do |
|---|---|---|
| Exact duplicate | Same supplier and same invoice number | Stop it. Do not pay until someone confirms it is not a duplicate. |
| Possible duplicate | Same supplier and same amount within a short window, such as 30 days | Hold it for a quick look. Often it is a duplicate; sometimes it is a genuine second order. |
Before comparing invoice numbers, tidy them up: ignore spaces, dashes, capital letters and leading zeros. That way "INV-01043" and "inv1043" are recognised as the same number.
For vendors, look beyond the name. Two vendor records with the same tax ID, the same address or the same bank details are probably one supplier set up twice.
Why shouldn't you block recurring same-amount charges?
Some charges are meant to repeat. Rent, software subscriptions, equipment leases, monthly service contracts and retainers arrive every month for exactly the same amount.
If your duplicate rule blocks every one of them, two things happen. First, someone has to clear the same false alarm every month. Second, people learn to click past duplicate warnings without reading them, and that is when a real duplicate gets through.
A better approach is a note: "Regular monthly charge, same amount as last month." The invoice still goes through review, but nobody treats it as a problem. What you still check is the period. Two rent invoices for October is a duplicate, even though one rent invoice a month is normal.
What should you check before paying an invoice?
Run this before an invoice goes into a payment run.
- The invoice number, tidied up, is not already in your system for this supplier
- No other invoice from this supplier for the same amount in the last 30 days, or if there is one, it covers a different period or PO
- The document is an invoice, not a statement, a reminder or a quote
- If it says "corrected", "revised" or "replaces", the original has been voided or credited
- The PO it refers to has not already been fully invoiced
- The supplier is set up once in your vendor master (search by tax ID and bank details, not only by name)
- The payment run itself does not pay the same supplier the same amount twice
Most of this takes seconds per invoice if the data is already captured. It takes much longer when every invoice number has to be looked up by hand.
What do you do when you find a duplicate invoice?
Before payment:
- Do not delete it. Mark it as a duplicate and link it to the original, so there is a record of what arrived and why it was not paid.
- If the supplier sent it again because they think they have not been paid, check the original's status and reply with the payment date.
- If it turns out to be a genuine second charge, clear the flag with a short note saying why.
After payment:
- Ask the supplier for a refund or a credit against their next invoice, in writing.
- Record the duplicate payment and how it was recovered.
- Look at the same supplier's recent history. If one duplicate got through, the same cause may have let others through too.
How do you stop duplicates at the source?
Detection catches duplicates. A few habits mean fewer of them arrive in the first place.
- One way in. Ask suppliers to send invoices to a single AP email address, and not to post paper copies as well.
- One record per supplier. Before creating a new vendor, search by tax ID and bank details. Merge duplicates you already have.
- Don't pay from statements. Use statements to reconcile, and pay only from invoices.
- Ask for credit notes, not replacement invoices. When a supplier corrects an invoice, a credit note against the original keeps the trail clean.
How SensiDocs helps
SensiDocs reads vendor invoices, receipts, credit notes and vendor statements in its Accounts Payable pack. It flags possible duplicate invoices, meaning the same supplier and invoice number, or the same amount within 30 days, and it notes regular same-amount charges so they are not mistaken for a problem. It also checks that lines, tax and charges add up to the total and that the PO number and remittance details are present. Every flag comes with a plain reason, and every account starts with 5 free results.