Duplicate Integer in Array

Given an array ARR of size N, containing each number between 1 and N-1 at least once, identify the single integer that appears twice.

Input:

The first line contains an integer, 'T', representing the number of test cases.
The first line of each test case contains a single integer, 'N', indicating the array size.
The second line contains 'N' space-separated integers denoting the elements of the array 'ARR'.

Output:

For each test case, return the duplicate number in the array. Output should be a single integer per test case, each on a new line.

Example:

Input:

ARR = [1, 2, 3, 4, 4]

Output:

4

Explanation:

The duplicate integer present in the array is 4.

Constraints:

• 1 <= T <= 10
• 2 <= N <= 10^5
• 1 <= ARR[i] <= N - 1
• A duplicate number is guaranteed to be present in the array.
• Time limit: 1 sec.