What it looks for
Four patterns, in order of how sure they are. The same invoice number paid more than once to the same supplier. The same invoice with its number written differently, such as INV-00412 and INV412. The same invoice number and amount under two supplier names. And the same amount to the same supplier twice within a few days.
The first two are marked as strong. The last two are marked as worth a look, because staged payments and regular charges can look the same.
What it does not do
It does not see credit notes, price lists, delivery notes or discounts, so it cannot find an invoice paid above the agreed price or a quantity that was never received. Those need the documents side by side, which is a bigger job.
A match is a question to ask the supplier, not proof of a mistake. Check each one against the invoice before you ask for money back.
Your figures stay with you
The rows you paste are read by this page inside your browser. They are not uploaded, stored or sent to me. The page records only that the tool was used, with no figures and no names. Close the tab and the rows are gone.
If you want the whole year checked
I can run the full check for you, including credit notes, price lists and quantities, and write the claims to suppliers. I am paid a share of what comes back, agreed in writing before I start. Use the contact link below and say roughly how many supplier invoices you pay in a month.