One number, two indices?

The input promises one valid pair. Which case makes this implementation reuse an index?

for (int i = 0; i < nums.length; i++) {
    seen.put(nums[i], i);
    int need = target - nums[i];
    if (seen.containsKey(need))
        return new int[]{seen.get(need), i};
}

Assume ordinary supporting code and valid inputs unless the question states otherwise.

Choose an answer before revealing the trace. Keys 1 to 3 work too.