How to make UNIQUE of a select field in a class definition?


I have a class “Post” with the following fields:

1)User_Id (Single Select Drop-down)
2)Title (Text)

I can easily make any text field as unique but in case of “Single Select”, Pimcore does not show “Unique” checkbox to make the field unique.
After double clicking on the field i.e. User_Id, it shows the checkbox but it does not work.

How can I make the field User_Id as Unique key?