Detect Duplicates in Player List | Dataford Interview Questions

FanDuel contest data may contain the same player more than once because of duplicated feed records or invalid lineup construction. Given a list of player records, return every duplicated player_id exactly once, ordered by the position where that ID is first detected as a duplicate.

Formal Specification

Implement find_duplicate_players(players), where players is a list of dictionaries. Each dictionary contains a hashable player_id, represented as an integer or string. Return a list of player IDs that occur at least twice. Each duplicate ID must appear once in the output.

A player ID is added to the result when its second occurrence is encountered. Do not mutate players.

Constraints

0 <= len(players) <= 10^5
Every record contains exactly one player_id key
Each player_id is an integer or string and is hashable
Preserve input order and do not modify the input list

Function Signature

def find_duplicate_players(players):

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Problem