Detect Duplicate Supplier Invoices in Google Sheets (Slack Alerts)

Stop paying the same supplier invoice twice — Detect duplicates in Google Sheets and notify your team on Slack

This n8n workflow runs every weekday morning to scan your supplier invoice Google Sheet, identify invoice pairs that appear to be duplicates (or the “same bill under a new number”), and post the results to Slack for review.

What this workflow does

• Scheduled check: A schedule trigger runs at 08:00 on weekdays.
• Reads your invoice tab once: The workflow pulls invoice rows from your Google Sheets invoice tab.
• Normalizes supplier names and invoice numbers: Supplier names are tidied (e.g., “Ltd”, “Limited”, and punctuation differences are ignored). Invoice numbers are compared in a way that treats “INV-000123” as matching “123”.
• Detects likely duplicate pairs: For the same supplier, it flags pairs when they match by:
  • Same invoice number + same amount
  • Same invoice number + different amount
  • Same amount under a new invoice number within a 45-day window
• Rescans safely: Any pair already present in your Possible duplicates tab is skipped, so previously reviewed items aren’t re-raised tomorrow.
• Writes new findings: New pairs are added to the Possible duplicates tab and sent as a single Slack message listing them.

Use cases

• Prevents duplicate payments before your payment run.
• Helps accounts payable teams catch revised invoice numbers and amount changes within a month.
• Reduces manual searching by centralizing review in a “Possible duplicates” Google Sheets tab and Slack alerts.

Technical details

• Integrations: Google Sheets and Slack
• n8n nodes/blocks: schedule trigger, google sheets, code, set, slack, and a sticky note for setup guidance
• Setup requirements: Add the Possible duplicates tab with the required 11 headers; paste your sheet URL in Settings; configure the four column names to match your invoice tab.

If the workflow flags too many pairs, you can lower dateWindowDays.