Duplicate numbers in an array

Write a function findDuplicates(numbers) that returns the duplicate numbers of an array: the numbers that occur more than once.

• numbers is an array of integers. They can be positive, negative or 0.
• Return a new array of numbers.
• Every duplicate number is in the result once, even if it occurs three or more times.
• Order: by the first occurrence of the number in numbers. [1, 2, 3, 2, 1, 5] gives [1, 2], not [2, 1]. Do not sort the result.
• If there are no duplicates (also for an empty array), return an empty array.
• Do not change the input array.

A set or a map helps you remember which numbers you have already seen.