Problem

In an Alten Nederland mobile app telemetry pipeline, event IDs should be unique within a single upload batch. Given a list of integers event_ids, return the first value that appears more than once when scanning from left to right. If no duplicate exists, return -1.

Formal Specification

• Input: event_ids, a list of integers
• Output: An integer representing the first duplicated value encountered during a left-to-right scan, or -1 if all values are unique

A value is considered the first duplicate if its second occurrence appears before the second occurrence of any other duplicated value.

Examples

Example 1

• Input: event_ids = [4, 7, 1, 7, 3, 1]
• Output: 7
• Explanation: 7 is the first value whose second occurrence is encountered.

Example 2

• Input: event_ids = [2, 5, 6, 8]
• Output: -1
• Explanation: No value appears more than once.

Constraints

• 1 <= len(event_ids) <= 10^5
• -10^9 <= event_ids[i] <= 10^9
• The solution should run efficiently for large input sizes

After implementing the function, discuss the time complexity of your solution and why it is better than checking every pair.