#include <cstdio>
#include <algorithm>
using namespace std;
int main(){
    int n;
    scanf("%d",&n);
    while(n--){
        int tt,a[500]={0},b[500]={0},sum=0;
        scanf("%d",&tt);
        for(int i=0;i<tt;i++){
            scanf("%d",&a[i]);
        }
        sort(a,a+tt);
        for(int i=1;i<tt;i++){
            if(a[i-1]==a[i])b[i]=b[i-1];
            else{
                b[i]=i;
            }
            sum+=b[i];
        }
        //printf("sum%d\n",sum );
        printf("%.6f\n",(double)sum/((double)tt*(tt-1)));
    }
    return 0;
}