Why is the service called twice in this angular 2 component?

I have here the component code, when I am subscribing to the observable the service is called twice, however if I subscribe to the Behaviorsubject it is only triggered once,

I can see on my logs that those are the result, please see my code below for my component the method subscribeToMap() method is called on ngOninit.

import { Component, OnInit } from '@angular/core';
import { Router }            from '@angular/router';

import { Observable }        from 'rxjs/Observable';
import { Subject }           from 'rxjs/Subject';

// Observable class extensions
import 'rxjs/add/observable/of';

// Observable operators
import 'rxjs/add/operator/catch';
import 'rxjs/add/operator/debounceTime';
import 'rxjs/add/operator/distinctUntilChanged';

import { HeroSearchService } from './hero-search-service';
import { Hero } from './../hero';

@Component({
  selector: 'hero-search',
  templateUrl: './hero-search.component.html',
  styleUrls: [ './hero-search.component.css' ],
  providers: [HeroSearchService]
})
export class HeroSearchComponent implements OnInit {
  heroes: Observable<Hero[]>;
  private searchTerms = new Subject<string>();

  constructor(
    private heroSearchService: HeroSearchService,
    private router: Router) {}

  // Push a search term into the observable stream.
  search(term: string): void {
    this.searchTerms.next(term);
    console.log("new " + term);
  }



  ngOnInit(): void {
    this.heroes = this.searchTerms
      .debounceTime(300)        // wait 300ms after each keystroke before considering the term
      .distinctUntilChanged()   // ignore if next search term is same as previous
      .switchMap(term => {
        return term   // switch to new observable each time the term changes
        // return the http search observable
        ? this.heroSearchService.search(term)
        // or the observable of empty heroes if there was no search term
        : Observable.of<Hero[]>([])})
      .catch(error => {
        // TODO: add real error handling
        console.log(error);
        return Observable.of<Hero[]>([]);
      });
      this.subscribeToMap();
  }

  subscribeToMap(): void{
     this.heroes.subscribe(() => console.log("called twice"));
     this.searchTerms.subscribe(() => console.log("called once"));
  }


  gotoDetail(hero: Hero): void {
    let link = ['/detail', hero.id];
    this.router.navigate(link);
  }
}

Here is the code for my service

import { Injectable } from '@angular/core';
import { Http }       from '@angular/http';

import { Observable }     from 'rxjs/Observable';
import 'rxjs/add/operator/map';

import { Hero }           from './../hero';

@Injectable()
export class HeroSearchService {

  constructor(private http: Http) {}

  search(term: string): Observable<Hero[]> {
    console.log("service is called");
    return this.http
               .get(`api/heroes/?name=${term}`)
               .map(response => response.json().data as Hero[]);
  }
}

thank you ver much!!!