Singly rendered content in Razor

HtmlHelperExtensions.cs

using System;
using System.Collections.Generic;
using System.Web;
using System.Web.Mvc;
using System.Web.WebPages;

public static class HtmlHelperExtensions
{
    public static HtmlString Once(this HtmlHelper @this, Func<dynamic, HelperResult> template)
    {
        string key = "Once";
        ISet<string> cache;
        if (@this.ViewContext.HttpContext.Items.Contains(key))
        {
            cache = (ISet<string>)@this.ViewContext.HttpContext.Items[key];
        }
        else
        {
            cache = new HashSet<string>();
            @this.ViewContext.HttpContext.Items[key] = cache;
        }
        string html = template(null).ToString();
        if (!cache.Contains(html))
        {
            cache.Add(html);
            return new HtmlString(html);
        }
        else
        {
            return null;
        }
    }
}

Once.cshtml

@for (int i = 0; i < 10; i++)
{
    @Html.Once(
        @<script type="text/javascript">
            alert("You will only see this once.");
        </script>);
}