Check if attribute value exist in before inserting

In my application, I there's an entity which I need to populate.   Preference is an Enum with value (one, two, three). While inserting a new object, I need to check, if the same preference has been given before, and if it is there, give the user a message. Right now, I've created a microflow on the save button where I'm getting the results from the database.   Any suggestions, how to change the hard-coded 'first' to the one user used while inserting the values?  Also, how to provide user an error if a value exisit, or insert if it doesn't