DB Table #__user_usergroup_map has two cols user_id and group_id
One user can be added multiple, because of multiple groups, but user_id is set as primary key.

Steps to reproduce the issue

Try import a #__user_usergroup_map where a user is associated to more aus one usergroup

you will see it runs into duplicate entry error
Duplicate entry ... for key 'PRIMARY'

Expected result

Should be possible to import

Actual result

Error
Duplicate entry ... for key 'PRIMARY'
Because of PRIMARY Key user_id could only be exists once
Same foru group_id

System information (as much as possible)

Joomla! 4.0.0-beta1 Beta [ Mañana ] 30-May-2020 18:32 GMT
Database Type | mysql 5.7.25-28