How to prevent drop down list item from repeating in vb.net?

How to prevent drop down list item from repeating in vb.net?

I'm using vb.net, I have something like this :
If textbox1.Text = "abc" Then
ddl1.Items.Add(1)
ddl1.Items.Add(2)
ddl1.Items.Remove(3)
ddl1.Items.remove(4)
End If
If textbox1.Text = "efg" Then
ddl1.Items.Add(3)
ddl1.Items.Add(4)
ddl.Items.Remove(1)
ddl.Items.Remove(2)
End If
How I'm going to make when I key-in "abc" more than one time its only add
once in drop down list menu and not repeated. same goes when I insert
"efg" ?
If I need to handle inside the dropdownlist_SelectedIndexChange how I'm
going to do it ?
Thank you for your concern.