C# MVC Lambda Distinct not working

I am unable to get a distinct list of 'Order' from my Lambda query. Even though am using the keyword Distinct() it is still retuing repeated select list item.

public ActionResult Index()
{
        var query = _dbContext.Orders
                                    .ToList()
                                    .Select(x => new SelectListItem
                                    {
                                        Text = x.OrderID.ToString(),
                                        Value = x.ShipCity
                                    })
                                    .OrderBy(y => y.Value);

        ViewBag.DropDownValues = new SelectList(query, "Text", "Value");

        retu View();
}

Any suggestions please?