Not quite sure what you mean by ".. already selected... cannot select that item again".
Two ideas that might help you achieve what you want.
1. Remove the item once selected - (perhaps moving it to another listbox).
2. Set SelectionMode to MultiSimple - that way once it is selected it stays selected until clicked again.
You have to be more specific as to what are you hoping to archive.
1. as Nick has said, you have an option of removing it from the list (if you don't want it to be selected twice), but what difference will it make if it's selected twice as the index will be the same, unless you have multiple items which are the same.
2. immediately after a selection has been made, disable the listbox until some condition has been met. lstbox.Enable=false; which you'll still have access to it programmatically