Problem

In Providence telemetry processing, an existing implementation checks every pair of event IDs to find the first duplicate, which is too slow for large inputs. Write an optimized algorithm that returns the index of the first event whose value has appeared earlier in the list.

Formal Specification

Implement a function first_duplicate_event(events) where:

• Input: events, a list of integers representing event IDs in arrival order
• Output: the index of the first duplicate event occurrence, or -1 if no duplicate exists

A duplicate is any value that has already appeared earlier. “First duplicate” means the duplicate occurrence with the smallest index.

Examples

Example 1 Input: events = [4, 2, 7, 2, 5, 7] Output: 3 Explanation: 2 first repeats at index 3. Although 7 also repeats, its duplicate occurs later at index 5.

Example 2 Input: events = [9, 1, 3, 4] Output: -1 Explanation: No event ID appears more than once.

Constraints

• 1 <= len(events) <= 10^5
• -10^9 <= events[i] <= 10^9
• Aim to improve on the naive O(n^2) approach
• Return only the duplicate occurrence index, not the value itself