Automatically attach user to external identity

Question

On SC 9.0.2 we want to map an existing user in sitecore automatically to an external identity provider, but get the following error:

[InvalidOperationException: Unable to create a user. Reason: DuplicateUserName]
   Sitecore.Owin.Authentication.Identity.MembershipUserStore`1.CreateAsync(TUser user) +475
   Microsoft.AspNet.Identity.<CreateAsync>d__0.MoveNext() +961
   System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw() +31
   System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task) +60
   Sitecore.Owin.Authentication.Services.<ResolveApplicationUserAsync>d__13.MoveNext() +949
   System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw() +31
   System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task) +60
   Sitecore.Owin.Authentication.Identity.<HandleFailureAsync>d__16.MoveNext() +548
   System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw() +31
   System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task) +60
   Sitecore.Owin.Authentication.Pipelines.Initialize.<<HandleExternalLoginCallbackUrl>b__10_1>d.MoveNext() +1341
   System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw() +31

We override the ExternalUserBuilder:

public class CustomExternalUserBuilder : Sitecore.Owin.Authentication.Services.ExternalUserBuilder
{
    public bool IsPersistentUser { get; private set; }

    public override ApplicationUser BuildUser(UserManager<ApplicationUser> userManager, ExternalLoginInfo externalLoginInfo)
    {
        var userName = $"custom\\{externalLoginInfo.ExternalIdentity.FindFirstValue("preferred_username")}";

        return new ApplicationUser(userName)
        {
            IsVirtual = !this.IsPersistentUser
        };
    }
}

and the UserAttachResolver:

public class CustomUserAttachResolver : UserAttachResolver
{
    public override UserAttachResolverResult Resolve(UserAttachContext context)
    {
        UserManager<ApplicationUser> userManager = context.OwinContext.GetUserManager();
        var user = userManager.FindByName(context.OwinContext.Request.User.Identity.Name);

        if(user != null)
        {
            return new UserAttachResolverResult(UserAttachResolverResultStatus.Attach);
        }

        return new UserAttachResolverResult(UserAttachResolverResultStatus.New);
    }
}

But allthough the CustomUserAttachResolver is added to the serviceConfigurator, it is never called. I tried to remove the existing UserAttachResolver, but I guess it's too late and that the DefaultApplicationUserResolver is already instantiated with the existing UserAttachResolver.