Implement a 'global uniq' (deduplication) function/algorithm as a 90-minute onsite coding exercise.
Asked in the onsite stage. Candidate attempted an in-degree-tree-based approach, which they felt was not optimal; ultimately rejected.
Implement global_uniq(items). The input is a list of hashable values, and the output must contain each distinct value exactly once, ordered by its first appearance. Do not modify the input list.
def global_uniq(items):