#include<cstdio>

#include<cmath>

#include<cstring>

#include<algorithm>

#define maxn 500005

using namespace std;

inline int getint()

{

    int num=,flag=;char c;

    while((c=getchar())<''||c>'')if(c=='-')flag=-;

    while(c>=''&&c<='')num=num*+c-,c=getchar();

    return num*flag;

}

struct cp{

    double a,b;

    cp(){}

    cp(double x,double y){a=x,b=y;}

    friend cp operator +(cp x,cp y)

    {return cp(x.a+y.a,x.b+y.b);}

    friend cp operator -(cp x,cp y)

    {return cp(x.a-y.a,x.b-y.b);}

    friend cp operator *(cp x,cp y)

    {return cp(x.a*y.a-x.b*y.b,x.a*y.b+x.b*y.a);}

};

const double pi=acos(-1.0);

int k=,bit;

cp a[maxn],b[maxn];

int rev[maxn];

long long num1[maxn],num2[maxn];

inline void fft(cp *a,int inv)

{

    for(int i=;i<k;i++)if(i<rev[i])swap(a[i],a[rev[i]]);

    for(int mid=;mid<k;mid<<=)

    {

        cp tmp(cos(pi/mid),inv*sin(pi/mid));

        for(int i=;i<k;i+=mid*)

        {

            cp ret(,);

            for(int j=;j<mid;j++,ret=ret*tmp)

            {

                cp x=a[i+j],y=ret*a[i+j+mid];

                a[i+j]=x+y,a[i+j+mid]=x-y;

            }

        }

    }

}

int main()

{

    int T=getint();

    while(T--)

    {

        memset(a,,sizeof a);

        int mx=;

        long long n=getint();k=,bit=;

        for(int i=;i<n;i++)

        {

            int x=getint();mx=max(mx,x);

            a[x].a++;

        }

        while(k<(mx+)*-)k<<=;

        while((<<bit)<k)bit++;

        for(int i=;i<k;i++)rev[i]=(rev[i>>]>>)|((i&)<<(bit-));

        for(int i=;i<k;i++)b[i]=a[i];

        fft(a,);

        for(int i=;i<k;i++)a[i]=a[i]*a[i];

        fft(a,-);

        for(int i=;i<k;i++)

        {

            num1[i]=(long long)(b[i].a+0.5),

            num2[i]=(long long)(a[i].a/k+0.5);

            if(!(i&))num2[i]-=num1[i>>];

            num2[i]>>=;

        }

        long long ans=(1ll*n*(n-)*(n-))/;

        for(int i=;i<=mx;i++)num2[i]+=num2[i-],ans-=num2[i]*num1[i];

        printf("%.7lf\n",1.0*ans*/(n*(n-)*(n-)));

    }

}