Duplicate invoice detection

# Duplicate invoice detection

## Purpose
Identify pairs of invoices that could be duplicates before they get paid twice. Two detection criteria: same exact amount from the same supplier within ±5 days, or same invoice number from the same supplier.

**Limitation:** detection is performed by Claude — there is no native deduplication in the API. The confidence score depends on the data available in get_payables.

## Instructions

### Step 1 — Retrieve payables for the period
```
Call get_payables for the period to check (e.g. last 3 months).
Retrieve for each payable: id, supplierId, amount, currency, invoiceDate,
invoiceNumber (if available), status.
Exclude payables of type creditNote.
```

### Step 2 — Detection by amount + supplier + date
```
Group payables by (supplierId, amount, currency).
For each group of 2+ payables:
  Check whether the invoice dates are within ± 5 days of each other
  If so → flag as "potential duplicate — identical amount"

Ignore subscriptions (payableType=subscription) — normal recurring payments.
```

### Step 3 — Detection by invoice number
```
If invoiceNumber is available in the metadata:
  Group by (supplierId, invoiceNumber)
  Any group of 2+ = confirmed duplicate (same reference)
  Flag as "probable duplicate — same invoice number"
```

### Step 4 — Enrichment of detected duplicates
```
For each suspicious pair:
  Call get_supplier_by_id to get the supplier's name
  Note: status (toPrepare, toExport, exported, paid) — pairs already paid take priority
```

### Step 5 — Duplicate report
```
Present by risk level:

HIGH RISK: same invoice number from the same supplier
  → Supplier | Amount | Date 1 | Date 2 | Status 1 | Status 2

MEDIUM RISK: same amount ± 5 days from the same supplier
  → Supplier | Amount | Date 1 | Date 2 | Days apart

For each pair: recommendation (block, verify, ignore)
```

## Points to watch
- The same amount from the same supplier ±5 days can be legitimate (e.g. weekly partial deliveries)
- Recurring subscriptions trigger false positives — exclude them explicitly
- This skill is a decision-support tool, not automatic deletion
- Prioritize pairs where at least one invoice is "toPay" or already exported

## Example prompts
- "Are there any duplicate invoices over the last 3 months?"
- "Check if we have duplicate invoices from Amazon or Salesforce"
- "Before paying this batch, identify potential duplicates"