using System;

using System.Linq;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        var artifacts = new List<Artifact>()

        {

            new Artifact { ID = 1 },

            new Artifact { ID = 1 },

            new Artifact { ID = 1 },

            new Artifact { ID = 2 },

            new Artifact { ID = 2 },

            new Artifact { ID = 3 },

        };

        List<int> duplicateArtifactIDs = GetDuplicates(artifacts.Select(a => a.ID));

​

        if (!duplicateArtifactIDs.Any())

        {

            return;

        }

        List<Artifact> duplicateArtifactsOriginal = artifacts.Where(x => duplicateArtifactIDs.Contains(x.ID)).ToList();

        List<Artifact> duplicateArtifactsFix = artifacts.Where(x => duplicateArtifactIDs.Contains(x.ID)).Distinct(new ArtifactComparer()).ToList();

        var duplicateArtifactsJake = new List<Artifact>();

        foreach (int duplicateArtifactID in duplicateArtifactIDs)

        {

            Artifact duplicateArtifact = artifacts.First(a => a.ID == duplicateArtifactID);

            duplicateArtifactsJake.Add(duplicateArtifact);

        }

        Console.WriteLine("duplicateArtifactsOriginal count: " + duplicateArtifactsOriginal.Count());

        Console.WriteLine("duplicateArtifactsFix count: " + duplicateArtifactsFix.Count());

        Console.WriteLine("duplicateArtifactsJake count: " + duplicateArtifactsJake.Count());

    }

    private static List<T> GetDuplicates<T>(IEnumerable<T> collection)

    {

        List<T> collectionList = collection.ToList();

        IEnumerable<T> uniqueItems = collectionList.Distinct();

        if (uniqueItems.Count() == collectionList.Count)

        {

            return new List<T>();

        }

        // We could do this right off the bat, but would rather do this only if necessary since

        // it's about 3x slower than just calling Distinct()

        List<T> duplicateItems = collectionList

            .GroupBy(i => i)

            .Where(group => group.Count() > 1)

            .Select(group => group.Key)

            .ToList();

        return duplicateItems;

    }

    public class Artifact

    {

        public int ID {get; set; }  

    }

    public class ArtifactComparer : IEqualityComparer<Artifact>

    {

        public bool Equals(Artifact x, Artifact y)

        {

            return x.ID == y.ID;    

        }

        public int GetHashCode(Artifact obj)

        {

            return obj.ID.GetHashCode();    

        }

    }

}