How do you stop paying the same invoice twice?

Why duplicates get paid

The same bill comes in through two doors. A vendor emails the PDF and mails a paper copy. A project manager forwards an invoice you entered last week. A vendor sends a 'reminder' that is really the original invoice again. Each copy looks legitimate on its own, and in a busy week both get paid.

What an exact match catches

Matching on vendor, invoice number and amount catches the common case: one bill entered twice. It depends on the invoice number being read correctly, so the check is only as good as the extraction behind it. A misread 1O42 instead of 1042 lets a duplicate slip through.

What it misses

A vendor who reissues the same charge under a new invoice number won't match. Neither will a copy with the amount keyed slightly differently. For these, sort open bills by vendor and amount once a week and look for pairs dated close together.

Watch the other direction too. Two identical monthly retainers, or a deposit and a final bill for the same round figure, can look like duplicates and aren't. Confirm a flagged pair is the same charge before you delete one.