ABC206_C.cs

using System;
using System.Collections.Generic;
using System.Linq;

class Program {
  
static void Main() {
    int n = int.Parse(Console.ReadLine());
    int[] a = Console.ReadLine().Split().Select(i => int.Parse(i)).ToArray();

    var d = new Dictionary<int, int>();
    foreach (int ax in a) {
        if (d.ContainsKey(ax)) {
            d[ax] += 1;
        } else {
            d.Add(ax, 1);
        }
    }
    long dup = 0;
    foreach ((_, int count) in d) {
        if (count > 1) {
            dup += (count * (count - 1) / 2);
        }
    }
    long all = n * (n - 1) / 2;
    Console.WriteLine(all - dup);
  }
}

7/14しか通らないが原因がわからない