KSeF error 440: duplicate invoice
KSeF (Poland's national e-invoicing system) rejects an invoice with error 440 when it already holds one with the same three values. Pick the XML files you are about to send, or a ZIP of them, to find repeats among them first.
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What counts as a duplicate
| # | Value | FA(3) element | Official wording (Polish) |
|---|---|---|---|
| 1 | Seller's NIP | Podmiot1/DaneIdentyfikacyjne/NIP | NIP sprzedawcy (Podmiot1:NIP) |
| 2 | Invoice type | Fa/RodzajFaktury | Rodzaj faktury (RodzajFaktury) |
| 3 | Invoice number | Fa/P_2 | Numer faktury (P_2) |
The rules around it
| Rule | Official wording (Polish) |
|---|---|
| Duplicates are detected across the whole system | KSeF wykrywa duplikaty faktur globalnie, w oparciu o dane przechowywane w systemie. |
| The error code is 440 | W przypadku duplikatu zwracany jest kod błędu 440 („Duplikat faktury”). |
| Uniqueness is kept for 10 full years after the year of issue | Unikalność faktury utrzymywana jest w KSeF przez okres 10 pełnych lat, liczonych od końca roku kalendarzowego, w którym faktura została wystawiona. |
| The criterion always refers to the seller; units issuing for one seller must agree on numbering | Kryterium unikalności odnosi się zawsze do sprzedawcy (Podmiot1:NIP). W przypadku, gdy w imieniu tego samego podmiotu faktury wystawiają różne jednostki (np. oddziały, jednostki organizacyjne JST, inne uprawnione podmioty), muszą one uzgodnić zasady numeracji, aby uniknąć duplikatów. |
Worked example
| File | Seller NIP | RodzajFaktury | P_2 | By the rule above |
|---|---|---|---|---|
| a.xml | 5251000201 | VAT | FV/2026/03/0142 | first of its combination |
| b.xml | 5251000201 | VAT | FV/2026/03/0142 | same three values as a.xml: duplicate |
| c.xml | 5251000201 | KOR | FV/2026/03/0142 | the type differs, so the combination differs |
| d.xml | 6793000422 | VAT | FV/2026/03/0142 | the seller differs, so the combination differs |
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Questions
Which fields make an invoice a duplicate in KSeF?
Three together: the seller's NIP (Podmiot1/NIP), the invoice type (RodzajFaktury) and the invoice number (P_2).
Does the buyer matter for error 440?
No. The documentation says the uniqueness criterion always refers to the seller. Two invoices with the same number and type from one seller are duplicates even when the buyers differ.
For how long is an invoice number blocked?
For 10 full years, counted from the end of the calendar year in which the invoice was issued.
Can this page tell me whether KSeF already holds my invoice?
No. It compares only the files you pick with each other. Whether a number is already in KSeF can be seen only in KSeF.
Source: Weryfikacja faktury (KSeF documentation, dated 09.04.2026). Read 2026-11-10.