Problem with module declarations

Ionic Framework ionic-v3
3

Take some time and grok: https://angular.io/guide/styleguide

For example: https://angular.io/guide/styleguide#core-feature-module

Keep the entries in your AppModule to a minimum:

@NgModule({
  declarations: [ AppComponent ],
  imports: [
    BrowserModule,
    CoreModule,
    IonicModule.forRoot(AppComponent, {
      backButtonText: ''
    })
  ],
  bootstrap: [ IonicApp ],
  entryComponents: [ AppComponent ]
})
export class AppModule {}

Do the heavy lifting in your CoreModule (e.g., your singleton services):

@NgModule({
  imports: [
    CommonModule,
    IonicModule,
    AngularFireModule.initializeApp(ENV.firebase),
    AngularFirestoreModule,
    AngularFireAuthModule,

    ...

    ENV.production ? ServiceWorkerModule.register('/ngsw-worker.js') : []
  ],
  exports: [],
  declarations: [],
  providers: [
    AuthService,
    { provide: ErrorHandler, useClass: IonicErrorHandler },
    GeolocationService,

    ...

    { provide: LoggerService, useClass: ConsoleLoggerService },
    MapsApiLoaderService
  ]
})
export class CoreModule {
  constructor( private afs: AngularFirestore,
               @Optional() @SkipSelf() parentModule: CoreModule) {

    throwIfAlreadyLoaded(parentModule, 'CoreModule');

    const settings = { timestampsInSnapshots: true };
    afs.app.firestore().settings(settings);
  }
}
Build Error "Type .... is part of the declarations of 2 modules: AppModule