#include <iostream>
#include <vector>
 
using std::cin;
using std::cout;
using std::endl;
using std::vector;
 
int checkFreq(const vector<int>& a) {
    vector<int> freq(10001, 0);
    for (int num : a) {
        freq[num]++;
        if (freq[num] >= 3) {
            return num;
        }
    }
    return -1;
}
 
int main() {
    int t;
    cin >> t;
 
    for (int i = 0; i < t; i++) {
        int n;
        cin >> n;
        vector<int> a(n);
        for (int j = 0; j < n; j++) {
            cin >> a[j];
        }
        cout << checkFreq(a) << endl;
    }
 
    return 0;
}
 

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7
1
1
3
2 2 2
7
2 2 3 3 4 2 2
8
1 4 3 4 3 2 4 1
9
1 1 1 2 2 2 3 3 3
5
1 5 2 4 3
4
4 4 4 4