For now, stick to including all your services in
AppModule to avoid problems like that (there will be only one service instance of that type in the entire app).
In Angular 6 you can avoid all that with
providedIn: root, so you won’t need to import singleton services in any module anymore and they won’t be included in the initial bundle (main.js) if they are used only in lazy loaded modules:
This is both great both in terms of performance as well as maintenance.
Angular 6 will work only with Ionic 4 (still in alpha) tough.