When the Event is trigered clear the dropdownlist before you populate it.
List<string> lsItems = null;
DdlTableName.Items.clear()
if (ddlTableType.SelectedIndex == 1)
{
lsItems = ems.getTestScriptComponentsList(con);
foreach (var item in lsItems)
{
ddlTableName.Items.Add(item);
}
}
else if (ddlTableType.SelectedIndex == 2)
{
lsItems = ems.getTestScriptsList(con);
foreach (var item in lsItems)
{
ddlTableName.Items.Add(item);
}
}