#include<cstdio>
#include<iostream>
#include<cstring>
#include<algorithm>
using namespace std;
int t,n,a[305];
double result;
void work()
{
    scanf("%d",&n);
    for(int i=1;i<=n;i++)
        scanf("%d",&a[i]);
    int total=(n-1)*n;
    double sum=0;
    sort(a+1,a+n+1);
    for(int i=1;i<=n;i++)
        {
            int k=a[i];
            int f=i;
            while(a[i]==a[f]&&f<=n)f++;
            sum+=(n-f+1);
        }
    result=sum/total;
    printf("%0.6f\n",result);
}
int main()
{
    cin>>t;
    while(t--)
        work();
    return 0;
}