Singleton page- and blocktypes. A attribute and initializationmodule that limits page and block types to only be created once. http://world.episerver.com/blogs/Per-Nergard/Dates/2016/4/limit-block-and-page-types-to-be-created-only-once-updated/

Singleton.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;

namespace AlloyDemoKit.Business.DataAnnotations
{
    public class Singleton : Attribute
    {
    }
}

SingletonTypeInitialzableModule

using System;
using EPiServer;
using EPiServer.Core;
using EPiServer.DataAbstraction;
using EPiServer.Framework;
using EPiServer.Framework.Initialization;
using EPiServer.ServiceLocation;
using AlloyDemoKit.Business.DataAnnotations;

namespace AlloyDemoKit.Business.Initialization.CreateTypeOnlyOnce
{
    [InitializableModule]
    public class SingletonTypeInitialzableModule : IInitializableModule
    {
        public void Initialize(InitializationEngine context)
        {
            DataFactory.Instance.CreatedContent += Instance_CreatedContent;
            DataFactory.Instance.DeletedContent += Instance_DeletedContent;
        }

        public void Preload(string[] parameters) { }

        public void Uninitialize(InitializationEngine context)
        {
            DataFactory.Instance.CreatedContent -= Instance_CreatedContent; ;
            DataFactory.Instance.DeletedContent -= Instance_DeletedContent;
        }

        void Instance_CreatedContent(object sender, ContentEventArgs e)
        {
            var repository = ServiceLocator.Current.GetInstance<IContentTypeRepository>();

            if (repository != null)
            {
                var contentType = repository.Load(e.Content.ContentTypeID);

                Attribute[] attributes = Attribute.GetCustomAttributes(contentType.ModelType);
                
                foreach (var attr in attributes)
                {
                    if (attr is Singleton)
                    {
                        var writableContentType = (ContentType)contentType.CreateWritableClone();
                        writableContentType.IsAvailable = false;
                        repository.Save(writableContentType);
                    }
                }
            }
        }

        void Instance_DeletedContent(object sender, DeleteContentEventArgs e)
        {
            var contentRepository = ServiceLocator.Current.GetInstance<IContentRepository>();
            var contentTyperepository = ServiceLocator.Current.GetInstance<IContentTypeRepository>();

            foreach (ContentReference cr in e.DeletedDescendents)
            {
                var content = contentRepository.Get<IContent>(cr);

                if (content != null)
                {
                    var contentType = contentTyperepository.Load(content.ContentTypeID);

                    System.Attribute[] attributes = System.Attribute.GetCustomAttributes(contentType.ModelType);
                    foreach (var attr in attributes)
                    {
                        if (attr is Singleton)
                        {
                            var writableContentType = (ContentType)contentType.CreateWritableClone();
                            writableContentType.IsAvailable = true;
                            contentTyperepository.Save(writableContentType);
                        }
                    }
                }
            }
        }
    }
}