Prevent duplicate selection on drop down custom fields

Hi

I have a custom 'approver' record where I assign an employee as an approver for return authorisation cases.

For example, employee Bob Smith is responsible for any returns relating to pricing or sales.

If I create another custom return authorisation approver record, I want there to be a way to prevent the user selecting 'pricing' or 'sales' in the multiselect 'Type of Return' field since those two return types have already been allocated to Bob Smith.

Is there a way to show an alert if the user tries to select these?

Thanks