Primary identifier is already taken by another user in this feed SIS error

SIS User Loader shows many errors: "The primary identifier is already taken by another user in this feed". Why?

This integration profile does not support a situation in which a user appears more than once in a single import/synchronization.

Quoted from: https://developers.exlibrisgroup.com...ment/sis/synch

Please verify that users are included just once in the file, or group of files, which are picked up by the SIS job.