import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner in=new Scanner(System.in);
        int T=in.nextInt();
        for(int i=0;i<T;i++){
            int arr[]=new int[in.nextInt()];
            for(int j=0;j<arr.length;j++){
                arr[j]=in.nextInt();
            }
            int sum=0;
            int count=0;
            for(int j=0;j<arr.length-1;j++){
                for(int k=j+1;k<arr.length;k++){
                    if(arr[j]!=arr[k]){
                        count++;
                    }
                    sum++;
                }
            }
            sum*=2;
            System.out.printf("%.6f",count*1.0/sum*1.0);
            System.out.println();
        }
    }
}