Common elements of two lists

Write a function commonElements(listA, listB) that returns the elements that are in both lists: their intersection.

• listA and listB are arrays of strings. They can be empty and can have duplicates.
• Return a new array with every common element once, even if it occurs several times in one or both lists.
• Order: the order of the first occurrence in listA.
• Case matters: 'ann' and 'Ann' are different elements.
• If there are no common elements, return an empty array.

A Set helps to check fast whether a value is in a list and to remember what is already in the result.