// It seems that you have a reacurring properties in your object.
// You can either clean up your object and make sure you are not getting
// The properties multiple times OR you can tell your serializer 
// That this is acceptable behavior like this:

var settings = new Newtonsoft.Json.JsonSerializerSettings();
// This tells your serializer that multiple references are okay.
settings.ReferenceLoopHandling = ReferenceLoopHandling.Ignore;

return JsonConvert.SerializeObject(, settings);