#include<cstdio>
#include<algorithm>
#include<cstring>
using namespace std;
int b[305]={0};
int main()
{
    int t,n,i,c;
    double ans=0;
    scanf("%d",&t);
    while(t--){
        ans=0;
        memset(b,0,sizeof(b));
        scanf("%d",&n);
        int a[n];
        for(i=0;i<n;i++){
            scanf("%d",&c);
            b[c]++;
        }
        for(i=0;i<305;i++){
            if(b[i]>=2){
                ans+=((double)b[i]/n)*(((double)b[i]-1.0)/(n-1));
            }
        }
        ans=(1.0-ans)/2.0;
        printf("%.6lf\n",ans);
    }
    return 0;
}