Unique form submissions in Kentico 12 MVC

using CMS;
using CMS.DataEngine;
using CMS.OnlineForms;
using DancingGoat.GlobalEvents;
using System.Linq;

[assembly: RegisterModule(typeof(FormGlobalEventsModule))]
namespace DancingGoat.GlobalEvents
{
    public class FormGlobalEventsModule : Module
    {
        public FormGlobalEventsModule() : base("DancingGoat.FormGlobalEventsModule")
        {
        }

        protected override void OnInit()
        {
            base.OnInit();

            // Register custom events
            BizFormItemEvents.Insert.Before += Insert_Before;
        }

        private void Insert_Before(object sender, BizFormItemEventArgs e)
        {
            var item = e.Item;

            // Sanity check to ensure we actually have a bizform item.
            if (item == null)
            {
                return;
            }

            // Get all column names that contain email.
            var columnNames = item.ColumnNames.Where(x => x.ToLowerInvariant().Contains("email"));

            // Loop over all column names containing the keyword email.
            foreach (var columnName in columnNames)
            {
                // Get the submitted email address value.
                var email = item.GetStringValue(columnName, string.Empty);

                // Lookup the current bizform for submissions with the same field value.
                var existingEntry = BizFormItemProvider.GetItems(e.Item.BizFormClassName)
                    .Column(columnName)
                    .TopN(1)
                    .WhereEquals(columnName, email)
                    .FirstOrDefault();

                // If it already exists, cancel it (displays an error on the MVC site).
                if (existingEntry != null)
                {
                    e.Cancel();
                }
            }
        }
    }
}