Create a function that takes an array of numbers and returns the number that's unique.
unique([3, 3, 3, 7, 3, 3]) ➞ 7
unique([0, 0, 0.77, 0, 0]) ➞ 0.77
unique([0, 1, 1, 1, 1, 1, 1, 1]) ➞ 0
Test cases will always have exactly one unique number while all others are the same.