List of values with job charges

Good afternoon

I have a list of values with job types in the company, but some jobs are unique and some are not. I would like that when a worker is assigned a unique position, he does not allow another to be assigned the same position, but if he is assigned a position that is not unique, it does allow several workers to use the same position.

Any idea how I can limit some values like that?

I appreciate any help.