A Key Field is a required field for the import and is designated with an asterisk on the import template. This issue is reported with this error message:
Cannot repeat key fields
Resolving the Import Error
If you receive this error, it is likely that you are 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 Total Line Items column. If not, add the column.
The Total Line Items column cells should be populated with the total amount of line items for a particular bill or invoice. For example, if there are three line items, the cells for that bill will be populated with the number 3. If there is only one line item, enter the number 1 in the cell for that row.