The error occurs when the CSV file contains duplicate data for a multi-select field column.
To resolve the error, review the CSV file, look for a multi-select field column, and make sure that no data (internal ID) is entered twice.
Example
Incorrect:
(CSV File) Custom Field value = 1|2|3|4|5|5|6
Correct:
(CSV File) Custom Field value = 1|2|3|4|5|6