K-th Duplicate Element in an Array

Question

## Problem

Find the k-th element that appears more than once, in order of first reaching a duplicate.

## Example
```
[1,2,1,3,2,1], k=2 -> 2
```

## Constraints
- 1 ≤ n ≤ 10^5

## Approach
Hash map of counts while scanning; emit on the k-th element crossing count 2. Reported HP question.