Remove Selected Item In Listbox

Remove an item in a listbox1 by clicking on it with the mouse to highlight it and then clicking the remove button

this is the code i have for button1_click


listbox1.Items.Remove(listbox1.SelectedItem)

How to check if a value already exist in list box before adding to it in asp.net

Dim xfind As ListItem = Nothing
xfind = SellerList.Items.FindByText(sellercnictxt.Text)

If xfind Is Nothing Then
   SellerList.Items.Add(sellercnictxt.Text)
End If