Avoid duplicate item in listview and show warning

I want to put a list of items in the listview and if I try to add one that is already there that shows me the text "duplicate", with what I have done does not add the duplicate and shows me the text "duplicate", if I add the non-duplicate but also shows me the text "duplicate"
any ideas?

thanks in advance

Hi David

First compare web.Text with the items in your List (For Each Loop). If found in List, notify, else add to List.

Could you please give me a clue as to which blocks to use?