title: Why observer method is called twice?
body: <p>I have the following method that returns observer, but it is called twice:</p>
  <pre class="lang-js prettyprint-override"><code>public getClassesAndSubjects(school: number, whenDate: string): Observable&lt;ClassesAndSubjects[]&gt; {
  
      console.log('1');
  
      const observable = this.classService.GetClassesAndSubjects(school, whenDate);
  
      console.log(&quot;2&quot;);
  
      observable.subscribe(data =&gt; {
          if (!data.hasOwnProperty('errors')) {
          this.classesSubjects = data;
  
          }
      }, error =&gt; {
          console.log(&quot;ERROR loading GetClassesAndSubjects: &quot; + error);
      });
  
      console.log(&quot;3&quot;);
  
      return observable;
  }
  </code></pre>
  <p>I mean a line <code>const observable = this.classService.GetClassesAndSubjects(school, whenDate);</code> is called twice and send two times request to server.</p>
  <p>Call this like:</p>
  <pre class="lang-js prettyprint-override"><code>this.classInstance.getClassesAndSubjects(school, date).subscribe(value =&gt; {
      // TODO 
  });
  </code></pre>