#include<iostream>
#include<cstdio>
#include<cstring>
#include<string>
#include<algorithm>
#include<cmath>
using namespace std;


int num[350];
int ans[350];
int sum;

int main ()
{
    int T;
    cin>>T;
    while(T--)
    {
        int n;
        cin>>n;
        for(int i = 0; i < n; i++)
            cin>>num[i];
        sort(num,num+n);
        sum = 0;
        ans[0] = 0;
        int tt = 0;
        for(int i = 1; i < n; i++)
        {
            if(num[i] == num[i-1])
            {
                ans[i] = ans[i-1];
                tt++;
            }
            else 
            {
                ans[i] = ans[i-1]+1+tt;
                tt = 0;
            }
            sum+=ans[i];
        }
        printf("%.6f\n", (double)sum/(n*(n-1)));
    }
    return 0;
}