When select drop down control dynamically while shown duplicate values again and again

Dear friends,
When i'm going to select drop down control dynamically while shown duplicate values again and again.
Here is my code:
SqlCommand cmd=new SqlCommand(" select did,depart from depatment",con);
SqlDataAdapter da=new SqlDataAdapter(cmd);
DataSet ds=new DataSet();
da.fill(ds);
ddlDepart.DataTextField="depart";
ddlDepart.DataValueField="did";
ddlDepart.DataBind();
ddlDepart.Items.Insert(0, new ListItem("--Select--","0"));
* If anybody knows Kindly support with me...
Thanks and regards,
Parthiban K