A Key Field is a required field for the import and is designated with an asterisk on the import template. If there's a duplicate key field, you'll encounter the following error:
Cannot repeat key fields
Resolving the Import Error
If you receive this error, you may be trying to import an invoice or bill with more than one line item. To resolve the issue, make sure that the last column on the CSV file is the Line Item Order column. If not, add the column.
The Line Item Order column cells should be populated with the number that determines the sequential order of the line items in the bill. For example, if there are three line items, the cells for that bill will be populated with the numbers 1-3. Enter a "1" for line item number 1, "3" for line item three, etc.