If the new item is already in the list, it will not be added.

When your goal is to calculate a sum of a list of numbers, the technique pointed out by @tylerboodman work.

If you need the literal list, including duplicates, such as for calculating a median or working with non-number values in some scenarios, you can try converting the list to text with join with, then adding the additional values using :append, and then converting the string back to a list using :split by.