Listbox items in WPF: strange behaviour

Question

Hi,
I need a little help with listboxes. I have created a listbox in WPF, and added items with the listbox1.items.Add(...) method from the c# code. The items appear alright, but I can't select them one-by-one, selection is all messed up. Seems that these items have the same index. How can I get listbox.items.add to work correctly?

Thank you in advance
V