Did we pay this twice?

Drop an AP export, a payment register or a bank CSV. This finds the payments that look duplicated and, just as importantly, leaves the rent alone. A duplicate finder that flags every subscription and every monthly retainer gets closed and never opened again.

Drop a CSV here, or click to choose one Read inside this tab. Nothing is uploaded, and there is no server to upload it to.

Days apart

No server, no account, no network calls once this page has loaded. Watch the Network tab if you like — this is a payment register, so you should.

What counts as suspicious

Four patterns, strongest first. Each group is scored by the money that would be recovered if it turned out to be real, because that is the order you want to work in.

The same invoice number twice. The strongest signal there is. An invoice number is supposed to be unique to the vendor, so seeing one twice usually means it was entered from both the emailed PDF and the posted copy.
Same vendor, same amount, days apart. The classic. Someone paid it, someone else did not see it, someone paid it again.
Same vendor spelled two ways. ACME Corp and Acme Corporation, Inc. are one vendor with two records, which is how the same bill gets paid from two different accounts. Names are compared with punctuation, case and the usual company suffixes removed.
Same amount, same day, different vendors. Weaker, and shown last, but it is how a payment mis-keyed to the wrong vendor shows up.

Why the rent is not flagged

The naive version of this tool flags every recurring payment, because rent is the same vendor and the same amount every month. That is the difference between a tool someone uses and a tool someone closes.

So before anything is reported, each vendor-and-amount group is tested for rhythm. Three or more payments spaced at a consistent interval of twenty days or more is a subscription, a retainer or a lease, and it is moved to a separate list you can glance at rather than a warning you have to dismiss. Two payments four days apart have no rhythm to speak of, and those are the ones worth your time.

The trade is deliberate and worth knowing: a genuine duplicate that happens to fall in a regular monthly series will be treated as part of the series. Suppressing the payments that are obviously fine is worth more than catching the rare duplicate that disguises itself as rent.

What it does not do

It reports; it never edits, and there is nothing to save. It cannot tell you a payment was actually duplicated, only that a pair has the shape of one — the invoice itself settles it. And a credit or reversal that offsets a duplicate is not detected, so a flagged pair may already have been put right.