Panorama 6 included No Duplicates and No Unique options, but they would only provide warnings, they did not prevent a user from entering any value into the database. This feature hasn’t been added to Panorama X yet, mostly because I haven’t decided how to present the warning. I’m leaning towards using a notification.
Instead of Gary’s suggestion of using the listchoices( function, I would suggest using the lookuplast( function. This function skips the current record, so it will reliably tell you whether a value appears in any other record besides the current record, which is what you want in this situation. You could also use the superlookup( function with the skipcurrentrecord option.
Keep in mind, though, that this will not prevent someone from entering a duplicate value through commands in the Morph menu, or by appending data. To fully enforce unique values you would have to lock down all of these alternate methods of modifying data.