MongoDB C# driver save fails because of duplication exception

mongodb.csharpdriver.save.fail.cs

using System;
using MongoDB.Bson;
using MongoDB.Bson.DefaultSerializer;
using MongoDB.Bson.IO;
using MongoDB.Bson.Serialization;
using MongoDB.Driver;
using MongoDB.Driver.Builders;
using NUnit.Framework;

namespace ContentFramework.UnitTests.Integration.MongoDb
{
    public class MyDocument
    {
        public string CompelId { get; set; }
        public string ContentDbId { get; set; }
        public string DbId { get; set; }
        public string Factory { get; set; }
        public string Locale { get; set; }
        public string ContentType { get; set; }
        public string[] Keys { get; set; }

        public string ParentId { get; set; }

        public override string ToString()
        {
            return String.Format("[CompelId:{0},Locale={1},CType={2};Keys:{3};ContentDbId={4}]",
                                 CompelId, Locale, ContentType, String.Join("-", Keys), ContentDbId
                );
        }
    }
    [TestFixture]
    public class MongoDbUpdate 
    {
        [Test]
        public void insert_and_update_document_using_save()
        {
            BsonClassMap.UnregisterClassMap(typeof(MyDocument));
            BsonClassMap.RegisterClassMap<MyDocument>(cm =>
            {
                cm.AutoMap();
                cm.MapIdProperty(x => x.DbId)
                .SetIdGenerator(new MyCustomIdGenerator())
                .SetSerializer(new MyCustomSerializer());
            }); 

            var server = MongoServer.Create("mongodb://localhost/?safe=true");
            var database = server["test"];
            var documents = database.GetCollection<MyDocument>("document");

            MyDocument document = new MyDocument();
            document.CompelId = "ssadfasdfasf";
            document.ContentDbId = "1";
            document.Factory = "testfactory";

            documents.Save(document);
            
            Console.WriteLine(document.DbId);

            document.Factory = "testfactory2";

            SafeModeResult safeModeResult = documents.Save(document, SafeMode.True);

            Assert.IsNotEmpty(document.DbId,document.DbId);
            Console.WriteLine(document.DbId);

            MyDocument doc = documents.FindOne(Query.EQ("_id", document.DbId));

            Assert.AreEqual("testfactory2",doc.Factory);
        }
        [Test]
        public void insert_and_update_document_using_update_query()
        {
            BsonClassMap.UnregisterClassMap(typeof(MyDocument));

            BsonClassMap.RegisterClassMap<MyDocument>(cm =>
            {
                cm.AutoMap();
                cm.MapIdProperty(x => x.DbId)
                .SetIdGenerator(new MyCustomIdGenerator())
                .SetSerializer(new MyCustomSerializer());
            }); 

            var server = MongoServer.Create("mongodb://localhost/?safe=true");
            var database = server["test"];
            var documents = database.GetCollection<MyDocument>("document");

            MyDocument document = new MyDocument();
            document.CompelId = "ssadfasdfasf";
            document.ContentDbId = "1";
            document.Factory = "testfactory";

            documents.Save(document);

            document.Factory = "testfactory2";

            Assert.IsNotEmpty(document.DbId, document.DbId);
            Console.WriteLine(document.DbId);

            documents.Update(Query.EQ("_id", new ObjectId(document.DbId)), Update.Set("Factory", "testfactory2"), SafeMode.True);

            MyDocument doc = documents.FindOne(Query.EQ("_id", new ObjectId(document.DbId)));

            Assert.AreEqual("testfactory2", doc.Factory);
        }

        public class MyCustomIdGenerator : IIdGenerator
        {
            public object GenerateId()
            {
                return ObjectId.GenerateNewId().ToString();
            }

            public bool IsEmpty(
                object id
            )
            {
                return string.IsNullOrEmpty((string)id);
            }
        }

        public class MyCustomSerializer : BsonBaseSerializer
        {
            public override object Deserialize(
                BsonReader bsonReader,
                Type nominalType,
                IBsonSerializationOptions options
            )
            {
                // read an ObjectId from the database but convert it to a string before returning it
                int timestamp, machine, increment;
                short pid;
                bsonReader.ReadObjectId(out timestamp, out machine, out pid, out increment);
                return new ObjectId(timestamp, machine, pid, increment).ToString();
            }

            public override void Serialize(
                BsonWriter bsonWriter,
                Type nominalType,
                object value,
                IBsonSerializationOptions options
            )
            {
                // convert the string to an ObjectId before writing it to the database
                var objectId = new ObjectId((string)value);
                bsonWriter.WriteObjectId(objectId.Timestamp, objectId.Machine, objectId.Pid, objectId.Increment);
            }
        }
    }
}