To get this clear you want to display the taxonomy terms of the child posts on the Parent Post template? Meaning you don't want to display the child posts just their taxonomy terms correct?
right. I have a Parent.CPT = Tatenbank-Fallbeispiel and related Child.CPT = Tatenbank-Massnahmen. I already have a View wich works fine. But I also want to have Loop of specific Taxonomy-Terms (Tatenbank-Schlagwörter) of the 3 related Child.CPTs in a Sidebar and also link them to the Archive. The problem is, that a view generates duplicate content (terms) because some of the related Child.CPTs have the same taxonomy.
Ok so this is the issue now, given that you are pulling the taxonomy terms based on the relationship data of the child post there will be repeats.
There isn't really anything we can do about this given the way view works. Essentially your specific case isn't achievable since you require the child post to display the terms and there are duplicates among them.