K-Different Pairs - Company-specific OAs / Twitter OA

Problems
1

Given an array of integers nums and an integer k, return the number of unique k-diff pairs in the array. A k-diff pair is an integer pair (nums[i], nums[j]), where both of the following are true:

This is a companion discussion topic for the original entry at https://algo.monster/problems/twitter_oa_k_difference/
2

Input: [1, 3, 6, 0], k=3
Output should be 2 ([3, 6], and [3, 0]), but the given solution says that the output should be 0.