I built an ajax chat in one of my mvc website. everything is working fine. I am using polling. At certain interval i am using $.post to get the messages from the db. But there is a problem. The message retrieved using $.post keeps on repeating. here is my javascript code and controller method.

 var t;        
        function GetMessages() {        
            var LastMsgRec = $("#hdnLastMsgRec").val();
            var RoomId = $("#hdnRoomId").val();
            //Get all the messages associated with this roomId
            $.post("/Chat/GetMessages", { roomId: RoomId, lastRecMsg: LastMsgRec }, function(Data) {
                if (Data.Messages.length != 0) {
                    $("#messagesCont").append(Data.Messages);
                    if (Data.newUser.length != 0)
                        $("#usersUl").append(Data.newUser);
                    $("#messagesCont").attr({ scrollTop: $("#messagesCont").attr("scrollHeight") - $('#messagesCont').height() });
                    $("#userListCont").attr({ scrollTop: $("#userListCont").attr("scrollHeight") - $('#userListCont').height() });
                }
                else {
                }
                $("#hdnLastMsgRec").val(Data.LastMsgRec);
            }, "json");


            t = setTimeout("GetMessages()", 3000);
        }

and here is my controller method to get the data:

public JsonResult GetMessages(int roomId,DateTime lastRecMsg)
        {
            StringBuilder messagesSb = new StringBuilder();
            StringBuilder newUserSb = new StringBuilder();            
            List<Message> msgs = (dc.Messages).Where(m => m.RoomID == roomId && m.TimeStamp > lastRecMsg).ToList();
            if (msgs.Count == 0)
            {
                return Json(new { Messages = "", LastMsgRec = System.DateTime.Now.ToString() });
            }            
            foreach (Message item in msgs)
            {
                messagesSb.Append(string.Format(messageTemplate,item.User.Username,item.Text));
                if (item.Text == "Just logged in!")
                    newUserSb.Append(string.Format(newUserTemplate,item.User.Username));
            }            

            return Json(new {Messages = messagesSb.ToString(),LastMsgRec = System.DateTime.Now.ToString(),newUser = newUserSb.ToString().Length == 0 ?"":newUserSb.ToString()});
        }

Everything is working absloutely perfect. But i some messages getting repeated. The first time page loads i am retrieving the data and call GetMessages() function. I am loading the value of field hdnLastMsgRec the first time page loads and after the value for this field are set by the javascript.

I think the message keeps on repeating because of asynchronous calls. I don't know, may be you guys can help me solve this.

or you can suggest better way to implement this.