Detect Duplicate Invoices in Google Sheets with Gemini + Slack

Detect Potential Duplicate Invoices in Google Sheets—Then Alert Slack and Place an ERP Hold (with Gemini)

This n8n workflow automatically checks newly added invoices in Google Sheets for duplicates using exact and fuzzy matching, classifies the result with Google Gemini, and then either alerts Slack and calls an ERP payment-hold API or logs a safe approval back to Sheets.

What this workflow does

• Triggers when a new invoice row is added to the “New_Invoices” sheet in Google Sheets.
• Loads invoice data from both “New_Invoices” and “Invoice_History”, then builds two comparison lists.
• Compares invoices using exact + fuzzy matching on invoice number, supplier name, amount, and invoice date to produce a duplicate flag, risk score, and best match details.
• Calls Google Gemini to classify each invoice as Exact Duplicate, Near Duplicate, or Legitimate Invoice, including confidence, reasoning, and a recommended action.
• If Gemini flags Exact Duplicate or Near Duplicate:
  • Posts a detailed alert to Slack (duplicate details and recommended action).
  • Sends a POST request to an ERP payment-hold endpoint to stop payment processing.
• If Gemini classifies it as legitimate:
  • Appends an approval record to the “Safe Invoices” sheet.
• Logs outcomes to the “Fraud Cases” sheet in Google Sheets (in all cases) for auditability.

Use cases

• Catch duplicate supplier invoices before they reach AP/ERP payment.
• Detect “near duplicates” caused by re-submissions, minor typos, or formatting differences.
• Centralize investigation history with Fraud Cases and Safe Invoices logging.

Technical details

• Integrations: Google Sheets, Slack, Google Gemini, and an ERP payment-hold API (POST endpoint).
• n8n nodes/logic: if, set, code, merge, slack, aggregate.
• Requires: Google Sheets OAuth with access to sheets/tabs: “New_Invoices”, “Invoice_History”, “Safe Invoices”, and “Fraud Cases”.