Prevent double email addresses appearing in all the mailinglists

Julian van der Horst
2019-05-22T20:49:47+02:00

Mailingslist can have duplicate emails.

### Current behaviour / Reproducing the bug
Steps to reproduce:

1. Create 2 users with duplicate email addresses in list for each membership
2. Check API Mailinglists
3. You two times the email

### Expected behaviour

It should contain only distinct emailadresses