ERR_DUPLICATE_TRANSACTION on QuickBooks: Duplicate transaction detected. Root cause: The system has detected that this transaction already exists, based on matching criteria such as the transaction reference number, amount, date, and counterparty. Duplicate detection prevents the same invoice, payment, or journal entry from being posted twice, which would distort financial records. Step 1: Search for duplicate. Find existing transaction Step 2: Compare transactions. Verify they are duplicates Step 3: Delete duplicate. Remove duplicate entry Step 4: Verify deletion. Confirm only one remains