#include<stdio.h>
#include<stdlib.h>
#include<iostream>
#include<string.h>
#include<cstring>
#include<string>
#include<math.h>
#include<algorithm>
using namespace std;
int Arr[310];
int main()
{
    int t=0,i=0,j,n=0,sum=0;
    double result=0;
    scanf("%d",&t);
    while(t--){
        scanf("%d",&n);
        sum=0;
        for(i=0;i<n;i++){
            scanf("%d",&Arr[i]);
        }
        sort(Arr,Arr+n);
        for(i=0;i<n;i++){
            for(j=i+1;j<n;j++){
                if(Arr[j]!=Arr[j-1]) break;
            }
            if(i!=(j-1)){
           ///     printf("%d %d\n",i,j);
                sum+=(j-i)*(j-i-1);
                i=j-1;
            }
            ///printf("%d\n",sum);
        }
        result=sum*1.0/(n*(n-1));
        printf("%.6lf\n",(1-result)/2);
    }
    return 0;
}