Would be interested to hear if anyone has an actual solution to this but I have settled on filtering out the extra posts by settings a flag in TempData using these extensions:

    public static class ControllerExtensions
    {
        private  static string GetRunFlagKey(ContentData block, string key)
        {
            return "RunFlag" + (block as IContent).ContentLink + key;
        }

        public static bool GetRunFlag(this Controller controller, ContentData currentContent, string key)
        {
            var b = controller.TempData[GetRunFlagKey(currentContent, key)] as bool?;
            return b.HasValue;
        }


        public static void SetRunFlag(this Controller controller, ContentData currentContent, string key)
        {
            controller.TempData.Add(GetRunFlagKey(currentContent, key), true);
        }
    }

In the controller's Post function I simply have to check if the post has been called already:

        [HttpPost]
        public ActionResult Index(myModel model, string myValue)
        {
            if (!this.GetRunFlag(model.CurrentBlock, myValue))
            {
                this.SetRunFlag(model.CurrentBlock, myValue);
                ....