#include <bits/stdc++.h>
 
typedef long double ld;
const int N = 1e6 + 50;
const int M = 2e5 + 50;
const ld pi = acos(-1.0);
 
int n, m, k, len, rev[N];
int yy[N], B[N];
struct Complex
{
    ld x, y;
    Complex(ld x = 0.0, ld y = 0.0) : x(x), y(y) {}
} A[N], omega[N], omegaInv[N];
 
inline Complex operator+ (const Complex &a, const Complex &b) { return Complex(a.x + b.x, a.y + b.y); }
inline Complex operator- (const Complex &a, const Complex &b) { return Complex(a.x - b.x, a.y - b.y); }
inline Complex operator* (const Complex &a, const Complex &b) { return Complex(a.x * b.x - a.y * b.y, a.x * b.y + a.y * b.x); }
inline Complex operator/ (const Complex &a, const ld t) { return Complex(a.x / t, a.y / t); }
 
inline void NTTInit(int n)
{
    k = 0, len = 1;
    while (len <= n) ++k, len <<= 1;
    for (int i = 1; i < len; ++i) rev[i] = (rev[i >> 1] >> 1) | ((i & 1) << (k - 1));
    omega[0] = omegaInv[0] = 1;
    Complex t(cos(2 * pi / len), sin(2 * pi / len));
    for (int i = 1; i < len; ++i) omega[i] = omegaInv[len - i] = omega[i - 1] * t;
}
 
inline void NTT(Complex *A, const Complex *w)
{
    for (int i = 0; i < len; ++i) if (i < rev[i]) std::swap(A[i], A[rev[i]]);
    for (int h = 1; h < len; h <<= 1)
        for (int t = len / (h * 2), j = 0; j < len; j += h * 2)
        {
            const Complex *wn = w;
            for (int i = j; i < j + h; ++i)
            {
                const Complex _1 = A[i], _2 = A[i + h] * *wn;
                A[i] = _1 + _2, A[i + h] = _1 - _2;
                wn += t;
            }
        }
    if (w == omegaInv)
    {
        for (int i = 0; i < len; ++i) A[i] = A[i] / len;
    }
}
 
inline char nc()
{
    static char buf[1000000], *p1 = buf, *p2 = buf;
    return p1 == p2 && (p2 = (p1 = buf) + fread(buf, 1, 1000000, stdin), p1 == p2) ? EOF : *p1++;
}
 
inline int read()
{
    int res = 0;
    char ch;
    do ch = nc(); while (ch < 48 || ch > 57);
    do res = res * 10 + ch - 48, ch = nc(); while (ch >= 48 && ch <= 57);
    return res;
}
 
 
inline void solve()
{
    n = read();
    memset(A, 0, sizeof A); memset(B, 0, sizeof B);
	memset(yy, 0, sizeof yy); 
    for (int i = 1; i <= n; ++i)
    {
        int t = read();
        ++yy[t];
        A[t].x = yy[t];
    }
    NTT(A, omega);
    for (int i = 0; i < len; ++i) A[i] = A[i] * A[i];
    NTT(A, omegaInv);
    long double ans = 0.0, sum = 0.0;
    for (int i = 0; i < len; ++i) B[i] = (A[i].x + 0.5);
    for (int i = 0; i * 2 < len; ++i) B[i * 2] -= yy[i];
    for (int i = 0; i < len; ++i) B[i] /= 2;
    for (int i = 0; i < len; ++i)
    {
        sum += B[i];
        ans += yy[i] * sum;
    }
    ans = 1 - ans / n / (n - 1) / (n - 2) * 2 * 3;
    printf("%.7Lf\n", ans); 
}
int main()
{
	NTTInit(M);
    int t = read();
    while (t--) solve();
}