Why does KSeF reject my invoice with error 440?

KSeF detected another invoice with the same seller NIP, invoice number, and issue date within a 10-year lookback window.

Was my original invoice actually submitted?

Probably yes. Query the KSeF status endpoint with your original reference number to confirm.

How do I correct an invoice already in KSeF?

Issue a correction invoice (faktura korygująca) that references the original KSeF number.

Can Invoice Navigator auto-fix this?

Invoice Navigator detects duplicate invoice numbers before submission and flags them.

Error codes · ksef · Poland

KSEF-440Partly

KSeF Duplicate Invoice Rejection (HTTP 440)

This is the rejection, word for word: „[KSEF-440] The KSeF system rejected your invoice because it detected a duplicate submission. KSeF checks the combination of seller NIP, invoice number, and invoice date against all invoices submitted in the last 10 years. If a match is found, the API returns HTTP status 440 and the invoice is not accepted.“

KSeF performs duplicate detection by hashing three fields: the seller's NIP (Numer Identyfikacji Podatkowej), the invoice number (P_2), and the invoice issue date (P_1). This check uses a 10-year lookback window. The FA(3) schema enforces these as mandatory fields, so the duplicate check always runs. The error is returned as an HTTP 440 status code from the KSeF REST API v2.0 endpoint /online/Invoice/Send.

What to have readyThe correct figures from your bookkeeping; we show you which field is affected.

What we doEnsure the invoice number (P_2) is unique per seller NIP and issue date combination. If this is a correction, use the correction invoice type with a reference to the original KSeF invoice number.

Choose the rejected invoiceor drop it here

XRechnung XML or ZUGFeRD PDF. Checking is free · corrected file €1.90 · the first every 30 days is free · no sign-up.

If you enter it yourself in your invoicing software

Before

<Fa>\n  <P_1>2026-03-25</P_1>\n  <P_2>FV/2026/03/0041</P_2>\n</Fa>

After

<Fa>\n  <P_1>2026-03-25</P_1>\n  <P_2>FV/2026/03/0042</P_2>\n</Fa>

What the finding looks likeExample

FindingNeeds your check · KSEF-440

From youThe correct figures

ThenChecked again

ProofSHA-256 and /verify link after the check

Often appears together with

KSEF-001KSeF NIP validation failed — bad format or wrong identifier field KSEF-010KSeF XML schema validation failed PL-R-003KSeF submission required