working with dropdownlists

Question

  i have taken two dropdownlists..i am populating id s in onw and names in another...if i select id in 1st dropdownlist the corresponding name should be displayed in 2nd dropdownlist....and vise versa....i have done that...but i am facing one problem...intially all values r populating are correctly..if i select anything that particular values is duplicating....i am unable to explain my probel exactly....can u tell me how to solve this...