Most people picture a duplicate payment as the exact same amount going out twice. It doesn't have to be. When the same vendor's invoice number shows up on two payments for different amounts, that is still worth a second look -- it just means something other than a clean, exact repeat, so this tool ranks it medium confidence and tells you what to check rather than calling it a duplicate outright.
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This is the exact pattern in the tool's own bundled worked example (click "Load a worked example" on the calculator to see it):
| Vendor | Invoice | Date | Amount |
|---|---|---|---|
| Cascade Freight Lines | INV-88120 | 2026-03-05 | $2,450.00 |
| Cascade Freight Lines | INV-88120 | 2026-03-09 | $245.00 |
Same vendor, same invoice number, four days apart -- but the second payment is exactly a tenth of the first. That specific ratio is a signature worth recognizing: a dropped decimal point or a mistyped digit produces exactly this kind of gap. The tool's own reasoning for this pair:
"Cascade Freight Lines: invoice INV-88120 was paid twice at different amounts ($2450.00 on 2026-03-05, $245.00 on 2026-03-09) — check for a partial payment or a data-entry error."
It does not tell you which one happened -- a genuine $245.00 partial payment and a $245.00 typo of a $2,450.00 bill produce the identical two rows in a CSV. Pulling the original invoice PDF is the only way to tell them apart.
Because the same vendor paying the same invoice number twice is unusual regardless of the amount -- a real invoice normally gets paid once. A different amount on the second payment means it is not a clean exact-duplicate (same invoice, same amount), so it is ranked medium confidence rather than high, and the tool tells you to check for a partial payment or a data-entry error rather than calling it a duplicate outright.
Cascade Freight Lines' invoice INV-88120 was paid $2,450.00 on 2026-03-05, then paid again, same invoice number, at $245.00 on 2026-03-09. That tenfold difference is exactly the kind of gap a dropped decimal or a mistyped digit produces -- worth checking against the original bill before assuming either payment was correct.
Yes -- a vendor invoice split across two payments (a deposit and a balance, for example) will also produce this exact pattern: same invoice number, two different amounts. The tool cannot tell a legitimate installment apart from a duplicate paid at the wrong amount from the CSV alone, which is why this tier is medium confidence and says to check, not high confidence saying to refund.
The exact-duplicate tier is the same vendor, same invoice number, and the same amount paid twice -- as clean a duplicate as a CSV can show, so it is ranked high confidence. This tier drops the amount match: same vendor and invoice number, but two different amounts, which could be a duplicate paid inconsistently, a partial payment, or a typo, so it is ranked one tier below and phrased as a question rather than a finding.
No. The CSV you check never leaves your browser -- there is no upload, no account, and no copy kept once you close the tab. Anonymous feature counts are local-only, contain no financial data, and can be turned off from the calculator's own page.