Items in a listbox are organized in an zero-indexed format, similar to the Python list. So, the first item resides in the first index, which is the number 0. The second item is at the number 1 ...

Python has several ways you can add or remove items from a list: .append() inserts an item at the end of the list. For example, list_of_ints.append(4) would turn list_of_ints into the list [1,2,3,4] .

def check_duplicate_improved(items): already_seen = {} for item in items: if item in already_seen: return True already_seen[item] = 0 return False. This means we only scan the main list once. We do ...