This article covers the duplicate student ID exception. Other import exceptions are in Managing Exceptions.
This exception means two rows in your data file have the same student ID number. If the rows don't contain identical data, the first row for that student imports and the second row causes the exception. The data in the first row displays in the application.
If the rows contain different data, you can concatenate them together. See the SIE FAQs.
If the rows are truly duplicates, remove the duplicate rows when you create the file. Your IT team needs to facilitate this process.