A "Key Field" is a required field for the import and will be designated with a * on the import template.
Cannot repeat key fields
This means there is a duplicate key field. This often happens when there is a large document.
Check the error report for the line of the error. Then start searching for the duplicate Key Field. This is most commonly the "name".
Once you find the duplicate, either change the cell or delete the row if it is a true duplicate.