ERR_BILL_DUPLICATE on Xero: Bill duplicate error. Root cause: Xero enforces a unique constraint on the combination of supplier, invoice number, and invoice date. When you attempt to create or import a bill with the same supplier invoice number and date as an existing bill — even if the amounts differ — Xero rejects it as a duplicate. This is a data integrity measure to prevent accidental double-posting of supplier invoices. The error can also occur when importing bills via CSV or API if the import process does not check for existing bills first. Step 1: Locate the existing bill in Xero. Go to Business > Bills > All Bills. Use the search box to filter by supplier name. Sort by Invoice Date (newest first). Look for a bill with the same supplier invoice number and date as the one you are trying to add. Note the bill status (Draft, Submitted, Approved, etc.) and the amount. If you find it, this is the duplicate. Step 2: Determine which bill is correct. Compare the duplicate bill to your source document (the PDF or email from the supplier). Check: (1) Is the invoice number identical? (2) Is the invoice date the same? (3) Is the total amount correct? If the existing bill in Xero has the wrong amount or is missing line items, it is likely a data entry error and should be deleted. If the existing bill is correct, you should not re-import it. Step 3: Delete the incorrect bill if it exists. Click the duplicate bill to open it. If the bill status is Draft, click Delete (trash icon, top right). If the bill status is Submitted or higher (Approved, Awaiting Payment), you must first click Undo to revert it to Draft status, then delete it. Note: Xero will ask for confirmation before deleting. Once deleted, the bill is moved to Xero's trash and can be recovered within 30 days if needed. Step 4: Check for import batch duplicates. If you are importing bills via CSV or API, check your import file for duplicate rows. Open your CSV in a spreadsheet application and sort by Supplier Name, then Invoice Number, then Invoice Date. Look for two rows with identical values in these three columns. Delete the duplicate row from your CSV before re-importing. Step 5: Re-submit or re-import the correct bill. If you deleted the wrong bill, manually re-enter it in Xero by going to Business > Bills > New Bill. Fill in the supplier, invoice number, invoice date, and line items exactly as they appear on the supplier invoice. If you are re-importing via CSV, upload the corrected file. Xero will now accept it because the duplicate has been removed. Step 6: Prevent future duplicates with bill matching. Enable Xero's bill matching feature to catch duplicates before they are submitted. Go to Settings > General Settings > Features and enable "Bill Matching". When enabled, Xero will alert you if a bill matches an existing purchase order or bill. Additionally, when importing bills, always use the "Match to PO" feature if your supplier invoices correspond to purchase orders — this prevents accidental duplicate entries.