Using a lookup to populate multiple fileds ( including employess ID ) on a form. How to verify that it is not a duplicate employees ID before OR after adding new item?

I have a lookup field named “Employee Info” that uses a lookup from a database. The main lookup comes as an underscored highlighted last name as a value ( I also opted use multiple fields tied to it such as Lastnamea and Firstname and employee ID.

I need to show the user if it is a duplicate.

I’ve tried using a workflow but I can’t seem to get it to work.

Is there any other way to determine if is a duplicate using for example, the employee id where if it is a duplicate set the value of field “status” to “duplicate”.

Thank you in advance.