How to show only entries with unique display names/values in Smart Form drop down field?

I need no-code solution to achieve this goal from any smart form masters - when the Smart Form's drop down list connected to SharePoint 2010 List column, the column could contain many different numbers or same numbers, how can I get drop down only shows unique display names... the  MS InfoPath 2010 can do this very easily (just check a box) ...Thanks for your helps!