Angular Service singleton constructor called multiple times

As you have declared the TestService as -

@Injectable({
  providedIn: 'root'
})

Which means you are adding to AppRoot module.

No need to add explicitly in the CoreModule, so remove from providers of CoreModule. Remove following -

providers: [
    TestService
  ]

As you are adding the TestSevice in CoreModule which is already added in RootModule that's the reason it constructor getting called multiple times.

So use either of one from above.