https://www.greatfrontend.com/questions/javascript/unique-array
Implement a function uniqueArray that takes in an array and returns a duplicate-free version of the array where only the first occurrence of each element is kept. The order of result values is determined by the order they occur in the array.
uniqueArray([1, 2, 3]); // [1, 2, 3]
uniqueArray([1, 1, 2]); // [1, 2]
uniqueArray([2, 1, 2]); // [2, 1]
