datavalue field problem in asp.net dropdown list

Hi I've a dropdown list with datasource like

empname     dept
-----------------
jax         02
martin      01
sachin      02

i've binded this table to my dropdown list and when i try to access dep开发者_高级运维t in dropdown selected event change everytime I'm gettin jax only. even i selected sachin.

Whats the problem? What I assume is I've two common values in my datavalue field (02 and 02) is this a problem?

Thank you

Be ensure that the data-binding code is executed once.

if(!IsPostBack)
 {
   //put data-binding code here
 }