var client = new MongoClient();
var database = client.GetDatabase("databasename");
var collection = database.GetCollection<Customer>("Customer");


var customerId = "..";
var builder = Builders<Customer>.Filter;
var filter = builder.Eq("Id", new ObjectId(customerId));
var matchedRecord = collection.Find(filter).SingleOrDefault();

// Insert new invoice into the customer record
matchedRecord.Invoices.Add(new Invoice {
  InvoiceNumber = "123456"
});

collection.InsertOne(matchedRecord);  // Error produced below

E11000 duplicate key error collection: databasename.Customer index: _id_ dup key: { : ObjectId('..') }

public class Customer
{
    public ObjectId Id { get; set; }

    public string CustomerId { get; set; }

    public List<Invoice> Invoices { get; set; }
}

public class Invoice
{
    public string InvoiceNumber { get; set; }
}

var builder = Builders<Customer>.Filter;
var filter = builder.Eq("Id", new ObjectId(customerId));

// Not sure how to use the FindOneAndUpdate method(?)
var matchedRecord = collection.FindOneAndUpdate<Customer>(filter);

var builder = Builders<Customer>.Filter;
var filter = builder.Eq("Id", new ObjectId(customerId));
var update = Builders<Customer>.Update.Set("Invoice.ClientName", "HELLO");

var update = Builders<Customer>.Update
            .Set("Invoices", new Invoice
            {
                ClientName = "HELLO"
            });

        var result = collection.UpdateOneAsync(filter, update);

var update = Builders<Customer>.Update.Push("Invoices", new Invoice
            {
                ClientName = "HELLO"
            });

var result = collection.UpdateOneAsync(filter, update);