Accounts payable control
Duplicate invoice detection: the seven types and how to stop them before payment
A duplicate invoice is the second entry of an invoice that was already recorded, and it is detected before payment by comparing each new invoice with the previous ones from the same supplier by number, amount, date and file. The risk is not the obvious resend of the same PDF, which any system spots, but the variants: the number written differently, the invoice that comes in through two channels or the one recorded in two companies of the group.
The seven types of duplicate invoice
| Type | How it happens | What gives it away | Difficulty |
|---|---|---|---|
| 1. Same file sent again | The supplier resends the e-mail or someone uploads the same PDF twice | The file fingerprint is identical | Low |
| 2. Same invoice through two channels | It reaches the invoices mailbox and the buyer's e-mail, and the buyer forwards it | Same supplier and same number, even if the file is different | Low |
| 3. Number written differently | “F-2026/0012”, “f 2026 0012” or “F20260012” depending on who types it or how it is read | The normalised number (upper case and without hyphens, slashes or spaces) matches | Medium |
| 4. Reprint or reminder | The supplier sends a “duplicate” or a payment reminder with the invoice attached | Same supplier, same amount to the cent, close dates | Medium |
| 5. Pro forma and final invoice | The pro forma is recorded and then the final invoice | Same amount and supplier; the pro forma should not have been recorded | Medium |
| 6. Duplicate across companies | The same invoice is recorded in two companies of the group | Recipient's tax ID (NIF): only one of the two companies is the recipient | High |
| 7. PDF and e-invoice of the same transaction | During the 2027 transition, the structured invoice arrives through a platform and its PDF by e-mail | Same supplier and same number in two formats | High |
Types 1 to 3 are channel errors; types 4 and 5, errors of judgement; types 6 and 7 come from the organisation (groups with a shared mailbox) and from regulation (formats coexisting).
What looks like a duplicate and is not
A rule that is too broad blocks legitimate payments and the team ends up ignoring the warnings. Three situations generate false positives:
- Fixed fees: rents, licences or maintenance contracts with the same amount every month. The amount matches, but the date and the number do not.
- Repeat purchases: the same weekly order from a supplies vendor, with the same amount two weeks in a row.
- Corrective invoices: they correct another invoice and quote it; they have their own number and are not a second copy.
That is why a matching amount on its own should warn, not hold, unless the date or the number also match.
Three worked examples (synthetic data)
Suppliers and invoices made up to explain the rules.
| Invoice already recorded | New invoice | What matches | What ininvoice does |
|---|---|---|---|
| Supplier A, no. F-2026/0012, EUR 3,480.00, 02/09 | Supplier A, no. f 2026 0012, EUR 3,480.00, 02/09 | Normalised number, amount and date | Holds the second one; a person decides |
| Supplier B, no. 5521, EUR 1,215.40, 10/09 | Supplier B, no. 5534, EUR 1,215.40, 24/09 | Amount to the cent within 14 days; different number | Warns without holding (it may be a repeat purchase) |
| Supplier C, no. 880, EUR 640.00, 15/09 | Supplier C, no. 881, EUR 655.00, 16/09 | Nothing relevant | Proceeds as normal |
In the first case, a system that compares the number as it is (“F-2026/0012” against “f 2026 0012”) would have accepted it and the company would have paid EUR 3,480.00 twice.
Does an invoice charge more than you ordered or received? The discrepancy checker matches its lines against the purchase order and the delivery note in your browser, free and with no signup. Open the discrepancy checker
The 2027 transition: the risk of the PDF plus the e-invoice
Mandatory e-invoicing between companies is not a PDF: Royal Decree 238/2026 requires a structured message in one of four syntaxes (UBL, CII, EDIFACT or Facturae). According to the Spanish Tax Agency, the obligation starts on 6 October 2027 for anyone whose volume of operations exceeds EUR 8 million and on 6 October 2028 for everyone else.
The same Royal Decree, in its second transitional provision, requires anyone above that EUR 8 million threshold, during the first twelve months, to accompany their e-invoices with a PDF document that guarantees their legibility, unless the recipient expressly accepts receiving them only in their original format, and that PDF is sent by any means, not through the public solution. In practice, for a year many invoices will arrive twice: the structured one through a platform and its PDF by e-mail.
If the two channels feed the same register without being cross-checked, the same invoice is booked twice. Three measures prevent it:
- Decide which channel is the original for each supplier and treat the other as a copy.
- Compare by supplier and normalised number, never by file: the XML and the PDF of the same invoice have different fingerprints.
- Expressly accept the original format with the suppliers that allow it, so that they stop sending the PDF.
Today ininvoice reads the invoice as PDF, image, scan or photo; reading e-invoices in XML is in development. The comparison by supplier and normalised number it already applies is the same one that will separate the PDF and the XML of the same invoice.
ininvoice's rules, one by one
| Match | Window | Result |
|---|---|---|
| Same file (same fingerprint) | Always | Not processed twice |
| Same supplier and same normalised number | No date limit | Holds the later invoice |
| Same amount and same number | No date limit | Holds the later invoice |
| Same supplier, same amount to the cent and same date, with no different lines | — | Holds the later invoice |
| Same supplier and same amount to the cent, another date, with no different lines | Less than 30 days | Warns without holding |
An invoice held as a possible duplicate is not discarded on its own: a person compares it with the original and decides. The amount-based rules do not pair invoices from different companies of the group, so that two identical purchases by two companies are not mistaken for a duplicate. ininvoice also validates the tax base, VAT, personal income tax withholding (IRPF), tax ID (NIF) and reverse charge, so an invoice with the right number and the wrong amounts does not pass as valid.
How to set up the control in four steps
- A single entry point per company or per group: one invoices mailbox, with buyers forwarding there instead of recording invoices themselves.
- Clean supplier data: one tax ID per supplier in the master file; two records for the same supplier hide duplicates between them.
- Rules that hold and rules that warn, as in the table above, so that warnings are not ignored because there are too many.
- Review the warnings before payment, not at audit: the practical guide on how to detect and prevent duplicate invoices includes the Excel formula to start with if there is no tool yet.
Frequently asked questions
How is a duplicate invoice detected?
By comparing each new invoice with those already recorded from the same supplier: same number (however it is written), same amount on close dates and the same file. None of those data points is enough on its own; the combination is what separates a duplicate from two identical legitimate purchases.
Why is it not enough for the ERP to reject repeated numbers?
Because the ERP compares the number as it was typed: “F-0012”, “F 0012” and “F0012” are different numbers to it. In addition, a duplicate can arrive with another number (a reprint, a reminder) or be recorded in another company of the group.
What does ininvoice do with a possible duplicate?
If the number from the same supplier matches, or the amount and the date match, it holds the invoice that arrived later and a person decides. If the amount matches on different dates within 30 days, it warns without holding. The same file is not processed twice.
Will the 2027 e-invoice put an end to duplicates?
Not entirely. During the first twelve months, anyone whose volume of operations exceeds EUR 8 million must accompany the e-invoice with a PDF that guarantees its legibility, unless the recipient expressly accepts the original format. If the PDF and the structured message come in through different channels, the same invoice can be recorded twice.
Can a duplicate payment be recovered?
Yes, by asking the supplier for a refund or offsetting it against future invoices, but it takes time and does not always succeed, especially with occasional suppliers. Stopping it before payment is much cheaper than recovering it afterwards.
A duplicate invoice should never reach payment
ininvoice holds an invoice matching another from the same supplier on number, or amount and date; if only the amount matches within 30 days, it warns. Send me 3–5 supplier invoices and we will go through them together.
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