If so then it is normal to get such an error if your link criteria is
not bound to a unique constraint, i.e. you are querying for something
that can be present in multiple records at the same time. In your case
you have 3 records which "end_date" field is NULL. When you get
multiple entries returned the AL gives you the opportunity to further
filter the records or to put them all in the resultant entry before
doing the input map. Since the AL does not know what exactly you might
want to be done you have been given the ability to specify your
intentions using a hook - "On Multiple Entries" to be precise. In that
hook you could cycle throughout all the entries that matched your
criteria using a script.