Angular 6 Creating Singleton Service

That's the default behaviour of adding providedIn to the service level. As per Docs

providedIn here tells Angular that the root injector is responsible for creating an instance of the HeroService. Services that are provided this way are automatically made available to the entire application and don't need to be listed in any module.

in your case just remove the providedIn: 'root' and have only under provides in the module.ts. Refer the following answer.

To avoid any other confusion you can create singletons in two ways, just as stated in the docs (singleton-services):

• Declare that the service should be provided in the application root.
  @Injectable({providedIn: 'root'}).
  The service should not be imported in any module then.
• Include the service in the AppModule or in a module that is only imported by the AppModule.
  Here no need to use {providedIn:'root'}.

In my case singleton services did not work when using JIT (normal ng build / ng serve). For me it only worked when using AoT (ng build --aot / ng serve --aot).