KSEF-440
Error Guide
🇵🇱 KSEF-440: KSeF Duplicate Invoice Rejection (HTTP 440) in Poland
ErrorMandatoryManual Fix
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.
Quick Facts
Severity
Error (rejection)
Ruleset
ksef
Fix Confidence
85%
Mandate
B2B Mandatory
Fix Type
Manual
Why This Matters in Poland
Poland requires e-invoicing using KSeF XML format. Invoices with this error will be rejected. They cannot be processed for Poland compliance.
Invoice Navigator covers 134 KSeF FA(3) rules for Poland.
How to Fix KSEF-440
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Example: Before & After
Before (invalid)
<Fa>\n <P_1>2026-03-25</P_1>\n <P_2>FV/2026/03/0041</P_2>\n</Fa>
After (fixed)
<Fa>\n <P_1>2026-03-25</P_1>\n <P_2>FV/2026/03/0042</P_2>\n</Fa>
Poland E-Invoicing Requirements
Phase
Status
Scope
KSeF voluntary
Live (Jan 2022)
All VAT payers (opt-in)
B2B mandatory — Phase 1
Live (Feb 1, 2026)
Large taxpayers (>200M PLN turnover)
B2B mandatory — Phase 2
Live (Apr 1, 2026)
All remaining VAT-registered entities (SMEs, sole proprietors, VAT-exempt businesses)
B2G
Live (Feb 1, 2026)
Included in general KSeF mandate
Financial penalties
Jan 1, 2027
Up to 100% of VAT amount on non-compliant invoices
Common Causes
- •Retry after network timeout — the original submission succeeded but your system did not receive the confirmation.
- •Invoice number reuse — the ERP system recycled or reset the invoice number sequence.
- •Test/production environment confusion.
- •Manual re-submission without checking status first.
Fix KSEF-440 automatically
Upload your invoice and let Invoice Navigator auto-remediate this error.