This comes from the original request for consistent aggregation of system report columns in MDL-76392 - creating this issue to also implement the option for displaying unique rows, with danmarsden describing the following use case:

The unique rows one is particularly useful when display course enrolment data when the user has multiple enrolment methods - eg manual enrolment and database enrolment - they show twice in the report unless you can turn unique rows on.

We have the same in custom reports per MDL-72826 (via a checkbox in the report editing form) - here we should have an API to achieve the same