Web Forms :: ListBox Binding Error - Select A Item From LB2 And Click On ADD The Selecteditem Is Added To LB3

I am having 3 ListBoxs(LB1,LB2,LB3).In LB1 I am binding the names of Courses through database.The code is shon below...

da1 = new SqlDataAdapter("SELECT COURSE_ID,COURSE_NAME FROM Course_Mst", con);
DataTable dt1 = new DataTable();
da1.Fill(dt1);
lstCourse.DataSource = dt1;
lstCourse.DataValueField = "COURSE_ID";
lstCourse.DataTextField = "COURSE_NAME";
lstCourse.DataBind();

Whenever I select particular course in LB1 all the content related to that course will be binded in LB2.The code is shown below.....

protected void LB1_SelectedIndexChanged(object sender, EventArgs e)
{
da2 = new SqlDataAdapter("SELECT SPEC_ID,SPEC_NAME FROM SPEC_TRANS WHERE COURSE_ID="+Convert.ToInt32(lstCourse.SelectedValue)+"", con);
DataTable dt = new DataTable();
da2.Fill(dt);
lstSpecilization.DataSource = dt;
lstSpecilization.DataValueField = "SPEC_ID";
lstSpecilization.DataTextField = "SPEC_NAME";
lstSpecilization.DataBind();
lstSpecilization.Focus();
}

I am having a ADD button.In LB2 I can select multiple items.When I select multiple items in LB2 and click on ADD all the selected items will be binded into LB3.The code is shown below...

protected void btnAddCourseDetaisl_Click(object sender, EventArgs e)
{
for (int intLoopIndex = 0; intLoopIndex <lstSpecilization.Items.Count ; intLoopIndex++)
{
if (lstSpecilization.Items[intLoopIndex].Selected)
{
ListItem li = new ListItem(lstSpecilization.Items[intLoopIndex].Text, lstSpecilization.Items[intLoopIndex].Value);
LB3.Items.Add(li);
}
}
}

My problem is when I select a item from LB2 and click on ADD the selecteditem is added to LB3 but if I select the same item and click on ADD the items are again added to the LB3.I want to prevent this.Pls respond me ASAP.