using System;

using System.Linq;

using Newtonsoft.Json.Linq;

public class Program

{

    public static void Main()

    {

        string json = @"

            [

              {

                ""nome"": ""Marcos"",

                ""pontos"": 12,

                ""acesso"": ""2016-04-22T19:10:00.2874904-05:00""

              },

              {

                ""nome"": ""Felipe"",

                ""pontos"": 12,

                ""acesso"": ""2016-04-22T19:10:00.2904923-05:00""

              },

              {

                ""nome"": ""Augusto"",

                ""pontos"": 15,

                ""acesso"": ""2016-04-22T19:10:00.2909925-05:00""

              },

              {

                ""nome"": ""Augusto"",

                ""pontos"": 12,

                ""acesso"": ""2016-04-22T19:10:00.2909925-05:00""

              }

            ]";

        Console.WriteLine("--- Test run #1 with a known duplicate ---");

        JArray array = JArray.Parse(json);

        CheckForDuplicateNomes(array);

        Console.WriteLine("\r\n--- Test run #2 with the duplicate removed ---");

        array.RemoveAt(3);

        CheckForDuplicateNomes(array);

    }

​

    private static void CheckForDuplicateNomes(JArray array)

    {

        try

        {

            // This will throw an exception if there are duplicate "nome" values.

            array.Select(jt => jt["nome"]).ToDictionary(jt => (string)jt);

            Console.WriteLine("No duplicates found!");

        }

        catch (Exception)

        {

            Console.WriteLine("There was at least one duplicate nome value in the array.");

        }

    }

}