#include<iostream>
#include<algorithm>
#include<iomanip>
using namespace std;
int a[302];
int main(){
    int i, j, T, n,total;
    cin >> T;
    while (T--){
        cin >> n;
        total = 0;
        for (i = 0; i < n; i++)
            cin >> a[i];
        sort(a, a + n);
        for (i = 0; i < n; i++){
            j = i + 1;
            while (j<n&&a[j] == a[i])
                j++;
            total += n - j;
        }
        cout << setiosflags(ios::fixed) << setprecision(6) << total*1.00 / n / (n - 1) << endl;
    }
}