How to make any field unique in detail form?

Hello,

I am creating a BP for overtime of employees.

All the employees who will have overtime will be added to the line items. The creator adds the same employee multiple time in the list which is not acceptable. I want to make the Employee Id (in the detail form) as a unique field.

Make any field unique on the upper form is available but I didn't see anywhere for detail form.

If anyone has any idea or you did this in past, please let me know.

Thanks

Fahad Yusuf