#include <iostream>
#include <cstdio>
#include <algorithm>
#include <cstdlib>
#include <cstring>
#include <string.h>
#include <string>
#include <map>
#include <queue>
#include <deque>
#include <cmath>
#include <math.h>
#include <vector>
#include <ctime>
#include <cctype>
#include <stack>
#include <set>
#include <bitset>
#include <functional>
#include <numeric>
#include <stdexcept>
#include <utility>
using namespace std;
#define ll long long 
#define m0(a) memset(a, 0, sizeof(a))
#define m1(a) memset(a, -1, sizeof(a))
#define C1(x)  cout<<x<<endl;
#define C2(x,y)  cout<<x<<" "<<y<<endl;
#define C3(x,y,z)  cout<<x<<" "<<y<<" "<<z<<endl;
struct stu
{
    int x,y,z;
    bool operator < (const stu &b) const
    {
        return x<b.x;    
    }
};
const int inf = 0x3f3f3f3f; 
const double eps = 1e-15;
const double pi = acos((double)-1) ;
const int maxn = 100100;
const ll M = 1e9+7;
ll i,j,k,l,m,n,x,y,z,ans,num;
int a[maxn]={0};
int v[maxn]={0};
int dx[4]={0,0,1,-1};
int dy[4]={1,-1,0,0};
int main()
{
    #ifdef ONLINE_JUDGE

    #else

        //freopen("in.txt","r",stdin);

    #endif
    int cas;
    scanf("%d",&cas);
    for (int casi=1;casi<=cas;casi++)
    {
        int x,y,z;
        double ans=0;
        m0(a);
        scanf("%d",&n);
        for (int i=1;i<=n;i++)
        {
            scanf("%d",&a[i]);
        }
        x=0;
        for (int i=1;i<n;i++)
        {
            for (int j=i+1;j<=n;j++)
            {
                if (a[i]!=a[j])x++;
            }
        }
        ans=double (x)/double (n*(n-1));
        printf("%f\n",ans);
        
    }
    return 0;
}