This question may seems duplicate but this is slightly different. In all other question in SO I had noticed that they have multiple routes registered. but in my case I have just one route. 

I am creating asp.net webapi (framework 4.5) and have just one route in RegisterRoutes() method - 

    public static void RegisterRoutes(RouteCollection routes)
    {
        routes.IgnoreRoute("{resource}.axd/{*pathInfo}");
 
        routes.MapRoute(
            name: "DefaultApi",
            url: "rest/{controller}/{id}",
            defaults: new { id = UrlParameter.Optional }
        );
 
    }

Then why is it throwing error? 

A route named 'DefaultApi' is already in the route collection. Route names must be unique. Parameter name: name