How to avoid paying duplicate invoices

Paying the same invoice twice is more common than it seems, and it is rarely caught in time. When it surfaces, it is usually at reconciliation, months later, or because the supplier flags it.

Why they slip through

• The supplier resends the invoice as a payment reminder and it is entered as a new one.
• The same invoice arrives through two channels: email and paper, or email and a portal.
• The number is read or typed differently: "1/567" versus "567", with or without leading zeros.
• A copy arrives followed by the original, or a corrected version without a clear credit note.
• Several people process invoices from the same inbox without knowing what the others have done.

Why checking the invoice number is not enough

Many ERPs block an invoice if another from the same supplier with the same number already exists. That only works if the number is always recorded exactly the same way. As soon as the format, a prefix or a slash changes, the check does not trigger.

Controls that actually work

• File fingerprint: if exactly the same PDF arrives, it is recognised instantly.
• Supplier, amount and date key: independent of the number, it catches resends even when the numbering changes.
• Normalised numbers: compared without prefixes, slashes or leading zeros.
• Human review only for doubtful cases: clear duplicates are blocked automatically; ambiguous ones are decided by a person.

One more rule: approved stays approved

If an already approved invoice arrives again, it must not go back to pending or lose the record of who approved it and when. Keeping that history is what makes the process auditable later.