Category affiliation: If a category affiliation gets changed from series to video in the 'other' event labels end up showing up mutliple times

[dagraf]

Steps to reproduce:

1. Have a series with at least two videos (A and B).
2. In video A: Create a category with at leas one label and set its affiliation to "Belonging to series".
3. In video B: Edit this category's affiliation to "Belongig to series". Click on save.
4. Still in video B: See that the labels get shown multiple times.
5. Still in video B: After a reload of the browser the labels do not get shown multiple times. But when opening the "Edit category" window the labels still show up multiple times.

After step 2 (see category "S - Teacher1"):

After step 4:

After step 5: