ERR_COPPER_CONTACT_DUP on Copper: Copper contact already exists. Root cause: Copper contact already exists error in Copper. Common causes: API errors, authentication issues, data validation failures, rate limiting, network problems. Step 1: Check error logs. Review integration logs to see exact error message. Step 2: Verify API credentials. Ensure API keys and tokens are valid and not expired. Step 3: Check API status. Verify the service is not experiencing outages. Step 4: Review request format. Ensure request body matches API specification. Step 5: Check rate limits. Verify you have not exceeded API rate limits. Step 6: Contact support. If problem persists, contact platform support with error details.