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first, my english is not so good(I'm from Germany)! -.- I hope you can understand what I mean.
I will build an app with a nickname. But I don't know how can I check whether is this nickname already used by an other user. The normal username field can't I use, because I need it for other things. And I have the same problem with another field.
Your English is fine so yes, we can understand your writing. However, I don't quite understand what you are asking about.
What do you mean that you want to build an app with a nickname?
The Users collection is a unique list that is associated with the app that you create. If you add a "nickname" field to the Users collection, if someone else with another app in the Kinvey backend is using that nickname, it will NOT conflict with your nickname.
So unless I am completely misunderstanding what you are asking, then you should be OK.
If I haven't understood your question correctly, can you please provide additional details about the use case you would like to implement?