VS 10 Avoid duplicates problem.

by the way .list1 from any other thread that helped doesnt work for my vs 2010 basic dont know why.
and also list

sample of my code.

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If TextBox1.Text = cd Then
ListBox1.Items.Add("alco")
ListBox1.SelectedIndex = ListBox1.Items.Count - 1 'for auto scroll it down'


End If

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so thats the textbox1.text, when i input cd on the texbox1
it automatically put "alco" on the listbox..

how can i avoid when i enter cd again on the textbox it doesnt allow it
to put it on the listbox1 since it has already got the alco in it..

ps: the textbox1.text is not my storage.. its just an input to put inside the listbox.1