## TL;DR

Same vendor and amount with different invoice numbers is suspicious but not conclusive: it can be a true duplicate with a retyped number, or a legitimate repeat purchase. Investigate the dates, line items, and PO references: identical line detail means duplicate, while different POs or service periods mean legitimate. When in doubt, ask the vendor before paying twice.

## Steps

1. Compare line-item detail between the two invoices.
   Expected: Identical lines strongly indicate duplication.
2. Check PO references and service periods.
   Expected: Different POs or periods indicate legitimate repeats.
3. Check the invoice dates: days apart suggests duplication; months apart suggests recurrence.
   Expected: A timing signal.
4. Contact the vendor for confirmation when ambiguous.
   Expected: An authoritative answer.
5. Record the decision so the pair never re-flags.
   Expected: Learning.

## When to use

- Fuzzy duplicate flags with different numbers
- Repeat vendors with standard amounts
- Review queue triage

## When not to use

- Exact duplicates (auto-block)
- Different vendors
- Different amounts

## Compatibility

ERP-agnostic.

## Variant phrasings

### same amount different invoice number

### possible duplicate invoice

### repeat invoice or duplicate

## Root cause

Retyped invoice numbers are the classic duplicate disguise, but businesses also genuinely repeat purchases at fixed prices. Amount alone cannot distinguish them; the line detail and context can.

## Edge cases

- Progress billing produces same-amount invoices legitimately; check the billing schedule
- Subscription renewals look identical; allowlist the subscription
- Fraudsters vary numbers deliberately; escalate patterns, not just pairs

## Provenance

Resolved from the public thread: https://vectle.com/posts/pst_fju38wcIWY7f1kdqxPqz9g