Spot invoices entered twice

# Duplicate invoice detection

## Purpose

Spot supplier invoices that may have been entered twice, **before** they are paid twice. Two signals: the same invoice number from the same supplier, or the same amount from the same supplier a few days apart.

There is no duplicate detection in the MCP: this is a comparison the assistant makes on the invoice list. The result is a list of pairs **to check**, not a list of confirmed duplicates. The skill deletes and blocks nothing.

## Step 1 — List the period's invoices

State the period (the last 3 months by default).

```json
{
  "tool": "get_invoices",
  "params": { "issuedAfter": "YYYY-MM-DD", "issuedBefore": "YYYY-MM-DD", "sortBy": "issueDate", "pageSize": 100 }
}
```

Each invoice already carries what is needed: `invoiceNumber`, `supplier.id` and `supplier.name`, `grossAmount`, `currency`, `issueDate`, `approvalStatus`, `paymentStatus`.

- With no approval filter, the list holds pending, approved **and rejected** invoices: keep the rejected ones, a duplicate has often been rejected once already.
- With no `type` filter, credit notes are not listed: that is intended, a credit note is not a duplicate.
- Page with `cursor` while `hasNextPage` is `true`; conclude nothing from a partial list.
- `grossAmount` can be `null` on a pending invoice: leave it out of the amount comparison.

## Step 2 — Same invoice number

Group by (`supplier.id`, `invoiceNumber`), comparing numbers regardless of case and spaces. Any group of two invoices or more is a **likely duplicate**.

To check one specific number across the whole history, the `invoiceNumber` filter does an exact search.

## Step 3 — Same amount, close dates

Group by (`supplier.id`, `grossAmount`, `currency`). In each group, flag invoices whose `issueDate` values are 5 days apart or less: **possible duplicate**.

Do not flag a supplier that bills the same amount every month (rent, subscription): the one-month gap already rules those out; say so if the user widens the window.

## Step 4 — Qualify each pair

For the pairs kept:

```json
{ "tool": "get_invoice_by_id", "params": { "id": "<invoice id>" } }
```

Read `payments` (has one of the two already been paid?), `attachments` (are the two documents the same file?) and `purchaseOrders`. This tool's payment status is authoritative for a given invoice.

Order of priority:

1. both invoices approved and unpaid — the double payment can still be avoided;
2. one paid, the other approved;
3. both paid — to flag for recovery.

## Report

```
Possible duplicates — invoices issued from <date> to <date> (N invoices checked)

Likely duplicates (same number): N pairs
| Supplier | No. | Amount | Date 1 | Date 2 | Payment 1 | Payment 2 |

Possible duplicates (same amount, 5 days or less): N pairs
| Supplier | Amount | Date 1 | Date 2 | Gap | Payment 1 | Payment 2 |
```

## Limits to state

- The same amount from the same supplier a few days apart can be legitimate (partial deliveries).
- Two different supplier records for one supplier escape the comparison: it works by supplier ID.
- The assistant cannot reject or delete an invoice: the decision is made in Spendesk.

## What was verified

The fields used were verified live on the demo account on 5 October 2026. The comparison itself was not replayed on all of the account's invoices.

## Example prompts
- "Are there any duplicate invoices over the last 3 months?"
- "Check for potential duplicates before my next payment run"
- "Was invoice INV-2026-7734 entered twice?"