Restrict user-list field

Hello,

I have lookup filed that contains list of users. On the same form I have one more field user-list drop down that contains all users in the app.

I need to filter users in second field to match the values in the first lookup field.
Lookup filed contains users (A,B), but second one contains users (A,B,C,D,I).

I want to either filter values in second field to only contain (A, B) or if filtering is not possible to write a formula to give an error if user selects a value that is not present in first field.

So for example:
(A), (B), (A,B) are valid choices
but (A, B, C), (A,C), (C) are not

Do you have maybe solution for this case?

Thank you in advance.

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Natasa Nedeljkovic
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