My code returns the correct answer the first duplicate is 3 (represented as int f) . I am struggling finding a more efficient way to find the first duplicate?

my constraints are 1 ≤ a.length ≤ 10^5, 1 ≤ a[i] ≤ a.length. Thanks.

class Program
{
  static void Main(string[] args)
  {
    int[] a = { 1, 2, 3, 4, 5, 6 };

    int f = FirstDuplicate(a);

    Console.ReadLine();
   }
public static int FirstDuplicate(int[] a)
{

    int[] answer = new int[2];
    answer[0] = -1;
    answer[1] = a.Length;
    for (int i = 0; i < a.Length - 1; i++)
        for (int j = i; j < a.Length; j++)
            if (a[i] == a[j] && i != j)
            {
                if (i < answer[1])
                {
                    answer[0] = a[i];
                    answer[1] = j;
                    break;

                }
            }

    return answer[0];
  }
}