Duplicate Invoice Detector: Catch Repeat Payments Before They Go Out
Screen every queued invoice against your history and the current batch — so the same bill never gets paid twice, and a person signs off on every flag.
A web tool where you import your queued invoice batch and your payment history, AI scores each invoice for duplicate likelihood and shows why, you confirm pay-or-hold on every flag, and it exports a cleared batch plus a duplicates-blocked log.
Before you start
- A Supabase account (free)
- A Vercel account (free)
- A Resend account (free)
- A CSV of your queued invoices and a CSV of your invoice/payment history
- Claude Code or any AI coding agent
The problem this kills
A vendor emails the same invoice twice. A coworker keys it under a slightly different spelling of the vendor name. A credit memo gets re-submitted as a fresh bill. A recurring charge sneaks through with a new invoice number but the same amount and date. Any one of these, and your company pays the same money twice — then spends weeks clawing it back, if it ever notices at all.
Duplicate payments are one of the quietest, most expensive leaks in accounts payable. The frustrating part is that they're almost always catchable: the clues are sitting right there in your invoice history and in the batch you're about to release. You just need something that checks every invoice against everything you've already paid — and against the rest of the batch — before the money leaves. You do not need to be a developer to build that something.
What you'll build
A simple internal web tool for your AP team. You import two files: the queued invoices you're about to pay, and your invoice/payment history. The tool scores every queued invoice for how likely it is to be a duplicate — catching exact repeats, the same invoice number hiding behind different vendor spellings, the same amount and date under a new number, and re-submitted credits. Each flag comes with a plain-English reason and a link to the invoice it matched. Your AP manager reviews each flagged invoice and clicks Pay or Hold. Only then is the batch released: the tool exports a cleared batch in your payment file's exact columns, plus a duplicates-blocked log of everything held and why.
What's inside the Implementation Plan
The downloadable plan is a step-by-step file you paste into an AI coding agent. It opens by interviewing you about your business — your AP process, the systems and spreadsheets you use, the exact columns and naming in your invoice data, your typical and peak batch sizes, and your real rules for what does and doesn't count as a duplicate. It reads a short spec back to you for a thumbs-up, then builds the tool around your answers instead of a generic template. From there it walks the agent through the data model, the CSV imports, the duplicate-scoring engine (exact and fuzzy), the review-and-decide screen, the human approval gate, and the cleared-batch and blocked-log exports. Every step ends with a ready-to-copy prompt.
The governance it includes (this is the point)
This isn't a toy. The plan builds in the controls a real finance function needs: login so only your team can use it, row-level security so people only see their own organization's invoices, a complete audit trail of every pay/hold decision and override (who, what, when, and why), a hard human-approval gate so nothing is released until a person decides, and duplicate guards so the same batch can't be imported and processed twice. The whole tool exists to make a careful human decision easy — the AI drafts the suspicion, a person makes the call.
Who it's for
AP managers, controllers, and accounting leads who own the payment run and are tired of finding double-pays after the fact. If you can describe what makes two invoices "the same" in your world, you can build this.
You've got this — open the plan, paste the first prompt, and you'll be scoring your first real batch for duplicates this afternoon.