What is the difference of "Core Cast" and "Customer Cast"

I recently noted that actors/caracters within the “Cast & Crew” menue were added/duplicated, thus screwing up my entire movie collector files. i wonder if ths has to do with the setting within the cast & crew tab. Can someone tell me

  1. how to prevent these unwanted changes?
  2. how to get ridd of these changes without going manually throu the entire file.

A fast and meaningfull answer is appreciated.

Core Cast is the automatic cast list that you get from our Core online movie database.
Custom Cast lets you EDIT the cast list, e.g. to add or remove actors.

I don’t think that is related.
Are you using Custom Cast lists at all?

Yes. I’m ONLY use customer cast since I add additional information to my listings. However, my files showed many of duplicated entries within my cast and crew tabs. for example:

my entry:
Baren Saat Saat, Baren *26.02.1984 Atiye Ösgürsoy (24 Episoden, 2019 - 2021)
added entry:
Baren Saat Saat, Baren *26.02.1984 Atiye Ösgürsoy

my entry:
Metin Akdülger Akdülger, Metin *10.04.1988 Ozan (21 Episoden, 2019 - 2021)
added entry:
Metin Akdülger Akdülger, Metin *10.04.1988 Ozan

I am interestedn in how to avoid these automatic/duplicated entries.
Can you help?

If you are using Custom Cast, and it has duplicates, then you must haver added these yourself. OR: you have been using Update from Core in the past, letting it adding cast members. That could cause duplicates.

Thx