Determining the uniqueness of element occurrence frequency in an array

Given a List named numbers containing integer values, return true if the count of each number's occurrences in the list is unique.

Examples:

numbers = [1] -> true (1 occurs 1 time) numbers = [2,2,1,1] -> false (2 occurs 2 times, 1 occurs 2 times ⇒ 2==2) numbers = [4,5,5,4,4,10] -> true (4 occurs 3 times, 5 occurs 2 times, 10 occurs 1 time ⇒ 3!=2!=1)