ASP.NET MVC 5 Identity - "A user name already exists" when using custom user

I am using ASP.NET MVC 5 to create a web app.

For the purposes of my app I have created a seprate "Website User" class that looks like this:

public class WebsiteUser
{
    public WebsiteUser()
    {
        this.Comments = new List<CommentEntityModel>();
    }
    public int Id { get; set; }
    public string Name { get; set; }
    public virtual ApplicationUser IdentityUser { get; set; }
    public ICollection<CommentEntityModel> Comments { get; set; }
}

My issue comes when I try and register a new user into the website.

My register action looks like this

public async Task<ActionResult> Register(RegisterViewModel model)
    {
        if (ModelState.IsValid)
        {
            var user = new ApplicationUser { UserName = model.Username, Email = model.Email };
            var websiteUser = service.CreateWebsiteUser(user);

            var result = await UserManager.CreateAsync(user, model.Password);
            var websiteUserResult = service.RegisterWebsiteUser(websiteUser);

            UserManager.AddToRole(user.Id, UserRoles.WebsiteUser.ToString());
            if (result.Succeeded && websiteUserResult)
            {
                await SignInManager.SignInAsync(user, isPersistent:false, rememberBrowser:false);

                return RedirectToAction("Index", "Home");
            }
            AddErrors(result);
        }

        // If we got this far, something failed, redisplay form
        return View(model);
    }

Pretty much the default one that Identity creates, except for these two lines:

var websiteUser = service.CreateWebsiteUser(user);
var websiteUserResult = service.RegisterWebsiteUser(websiteUser);

And what I have for them in the service is this :

CreateWebsiteUser()

public WebsiteUser CreateWebsiteUser(ApplicationUser appuser)
    {
        var user = new WebsiteUser
        {
            Name = appuser.UserName,
            IdentityUser = appuser
        };

        return user;
    }

RegisterWebsiteUser()

public bool RegisterWebsiteUser(WebsiteUser websiteUser)
    {
        try
        {
            this.context.WebsiteUsers.Add(new WebsiteUser {Name = websiteUser.Name, IdentityUser = websiteUser.IdentityUser});
            this.context.SaveChanges();
            return true;
        }
        catch (Exception e)
        {
            throw new Exception(e.Message);
        }

    }

And my issue is, that whenever I try to register a new user I get an exception on the this.context.SaveChanges() line, saying "A user name with the name |Name| already exists".

|Name| is basically the value of the UserName column in the ASPNetUsers Table.

I am really stuck and can't find a solution. (I can't even understand why an error like this would happen)

My DB structure seems okay to me so I guess the issue is somewhere else.

The only solution I can think of, is to completely remove my own User class and just add properties to the deafult ASP User.

Any help is greatly appreciated.