Import fails for non-numerical Matrikelnummers
Matrikelnummers are parsed as integer, so when it is not numerical, it defaults to 0. This means that all students with such Matrikelnummers will be combined into one.
The affected students can be found with this query:
select
c.id, c.name, s.id, s.firstname, s.lastname
from
courses c, groups g, groups_students gs, students s
where
g.course_id = c.id and
gs.group_id = g.id and
gs.student_id = s.id
group by
c.id, c.name, s.id, s.firstname, s.lastname
having count(*) > 1