#include<iostream>
#include<cstdio>
#include<cstring>
#include<algorithm>
#define N 1005
using namespace std;
int t[N],a[N];
int cmp(int a,int b)
{return a<b;}
int main()
{
 int ca;
 cin>>ca;
 while (ca--)
 {
  int n;
  cin>>n;
  memset(t,0,sizeof(t));
  for (int i=1;i<=n;i++)
  {
   scanf("%d",&a[i]);
   t[a[i]]++;
  }
  int cnt=0;
  for (int i=1;i<=300;i++)
   if (t[i]>=2){
                cnt+=t[i]*(t[i]-1);
                }
  int num=n*(n-1);
 double p=double((num-cnt)/2)/(double)(n*(n-1));
 printf("%.6lf\n",p);
 }   
 //system("pause"); 
}