Detecting an invoice with a number you have already paid is straightforward and every product does it. The duplicates that actually cost money are the ones that do not look like duplicates: a reissued invoice with a new number, the same charge appearing on a statement and an invoice, or a supplier paid once by the accounts team and once on somebody's card. Those need different checks.
The easy case, and its limits
Same supplier, same invoice number, already recorded. This catches the accidental resubmission and the double entry, which are common and cheap to prevent. It catches nothing where the number differs, and suppliers reissue invoices with new numbers routinely, particularly after a query or a correction, so this check alone leaves the expensive cases open.
The checks that catch the rest
Same supplier, same amount, similar date, regardless of number. Same purchase order line invoiced twice. An amount matching a recent payment made outside the payables run. Each produces false positives, which is why they should raise a query rather than a block, and why a person reviewing them needs enough context to judge quickly.
Prevention is cheaper than detection
Most duplicates originate in process rather than in fraud: an invoice emailed to two people and entered twice, or paid on a card and then invoiced. Consolidating arrival channels and requiring order references removes more duplicates than any detection rule, and the detection then handles the residue rather than doing all the work.
Questions people ask about duplicate payment software
How common are duplicate payments?
Common enough that most organisations of any size have made one, and they are usually recoverable if noticed. Measure your own rate rather than relying on a published figure.
Should detection block or warn?
Block on an exact match of supplier and invoice number. Warn on the fuzzier checks, because they produce false positives and a block would train people to override.
How do we recover a duplicate payment?
Ask the supplier, usually against the next invoice as a credit. Speed helps, which is another argument for detection rather than discovering it at a year-end reconciliation.