440: Duplicate invoice
What it means
KSeF invoice status 440 means KSeF already holds an invoice with the same seller NIP, invoice type and invoice number.
The usual cause
KSeF looks for duplicates across every invoice it holds, by the seller's NIP (Podmiot1), the type of invoice (RodzajFaktury) and the invoice number (P_2), for 10 full years from the end of the year of issue. This happens when the same invoice is sent twice, or when units that issue invoices for the same seller, such as branches, use the same numbering.
How to fix it
The details give the KSeF number and the session of the earlier invoice: check it first. If it is the same invoice, do not send it again; if it is a different one, give it a new number and send it.
Elements the check reads
-
Podmiot1/DaneIdentyfikacyjne/NIP -
Fa/RodzajFaktury -
Fa/P_2
The code as published
Duplikat faktury Read the official document . Source: KSeF API 2.0, specyfikacja OpenAPI (open-api.json), version 2.9.0, published by Ministerstwo Finansów, read on 8 November 2026.
Look at your file
Open the invoice in your browser to find the fields this code names: Open a Polish KSeF invoice. Open a Polish KSeF invoice