I think the key issue here is that you need to modify the association in your domain model first: the association between Vak and VakCategorie is 1..* which means that a Vak can only be associated with one VakCategorie. You need to change that to *..*.
The rule that a VakCategory can not contain duplicate Vakken need to be enforced using a validation microflow similar to what you show in the printscreens.
Does that put you on the right track?
Oh yeah ofcourse. Thnx, will be able to work with that.