Any ideas how to fix this?
UserService.UserServiceClient userServiceClient = new UserServiceClient();
userServiceClient.GetUsersCompleted += new
Not sure if it's really a problem, but I see you have the same name for your binding configuration ().
I usually try to call my endpoints something like "UserServiceBasicHttp" or something similar (the "Binding" really doesn't have anything to do here), and I try to call my binding configurations something with "....Configuration", e.g. "UserServiceDefaultBinding", to avoid any potential name clashes.
Marc