Problem

In a Meta event ingestion pipeline, you receive a stream of event records in arrival order. Write a function that removes duplicate records while preserving the order of the first occurrence of each unique event.

A record is considered a duplicate if its event_id has already appeared earlier in the stream. Return the filtered list of records in the same relative order as their first appearance.

Formal Specification

• Input: events, a list of event records
• Each event record is a dictionary with at least the key "event_id"
• Output: a list of event records containing only the first occurrence of each distinct event_id

Examples

Example 1

Input: events = [
  {"event_id": "a1", "user_id": 10},
  {"event_id": "b2", "user_id": 11},
  {"event_id": "a1", "user_id": 12}
]
Output: [
  {"event_id": "a1", "user_id": 10},
  {"event_id": "b2", "user_id": 11}
]

The second a1 is removed because a1 was already seen.

Example 2

Input: events = [
  {"event_id": 5},
  {"event_id": 5},
  {"event_id": 6},
  {"event_id": 5}
]
Output: [
  {"event_id": 5},
  {"event_id": 6}
]

Only the first occurrence of each event_id is kept.

Constraints

• 0 <= len(events) <= 10^5
• Each event is a dictionary containing event_id
• event_id is hashable
• Preserve the original order of first occurrences