PopUp alert message using Event receiver in sharepoint 2010

I have created event receiver with C# coding in SharePoint.when item should match is already have. it showing massage like this is my code..

    public override void ItemAdding(SPItemEventProperties properties)
    {
            base.ItemAdding(properties);  
            SPList oList = null;
            string strCarName = string.Empty;
            using (SPWeb web = properties.Web)
            {
                oList = web.Lists["Cars"];
                string dropdwnvalue = web.Lists[properties.ListId].Fields["CarName"].InternalName;
                String lookupFieldStringValue = Convert.ToString(properties.AfterProperties[dropdwnvalue]);
                SPListItem item = oList.Items[Convert.ToInt32(lookupFieldStringValue) - 1];
                strCarName = Convert.ToString(item["LinkTitle"]);
            }
            string strStartdate = Convert.ToString(properties.AfterProperties["EventDate"]);
            SPQuery existingItemsQuery = new SPQuery();
            existingItemsQuery.Query = "<Where><And><Eq><FieldRef Name='EventDate'/><Value Type='DateTime'>" + strStartdate + "</Value></Eq><Eq><FieldRef Name='Car_x0020_Name'/><Value Type='Lookup'>" + strCarName + "</Value></Eq></And></Where>";
            SPListItemCollection existingItems = properties.List.GetItems(existingItemsQuery);


                if (existingItems.Count >= 1)
                {

                    properties.Cancel = true;
                    properties.ErrorMessage = "Item is already exists";
                    properties.Status = SPEventReceiverStatus.CancelWithError;
                    // ClientScript.RegisterStartupScript(typeof(Page), "test", "<script>alert('Hello');return false;</script>"); 
                }

output:-

item is already exists.
that item adding only one time in a day.again i cant add another item. But i want add another item in a day change time

any one can help me out.