#include<iostream>
#include<cstdio>
#include<cstring>
#include<algorithm>
#include<cstdlib>
#include<cmath>
#include<set>
#include<queue>
#include<map>
using namespace std;

int n,a[10005];
int read(){
    int x=0,fu=1; char ch=getchar();
    while (ch<'0' || ch>'9'){ if (ch=='-') fu=-1; ch=getchar(); }
    while (ch>='0' && ch<='9'){ x=x*10+ch-'0'; ch=getchar(); }
    return x*fu;
}
int main(){
    int cas=read();
    while (cas--){
        n=read(); int i,j,ans=0;
        for (i=1; i<=n; i++) a[i]=read(); sort(a+1,a+n+1);
        j=1;
        for (i=1; i<=n; i=j){
            while (j<=n && a[i]==a[j]){ ans+=i-1; j++; }
        }
        printf("%.6f\n",(double)ans/n/(n-1));
    }
    return 0;
}