我试图创建一个表单来提交详细信息，但当我订阅服务时，订阅完成了两次。第一个是空数据，第二个是实际数据。
Component.ts

addBook() {
    this.errorMessage = null;
    this.fireService.getBook(this.bookDetails.isbn).subscribe(data => {
      console.log('subscribe called');
      if (null != data) {
        this.dbox.open(DialogBoxComponent, {
          data: { title: 'Error', content: 'Book is already present in Library. Select Edit book to modify the details', button: false }
        });
        this.errorMessage = 'Book is already present in Library. Select Edit book to modify the details';
      } else {
        this.fireService.addBook(this.bookDetails);
        this.dbox.open(DialogBoxComponent, {
          data: { title: 'Success', content: 'Book has been Added to Database', button: false }
        });
        this.router.navigateByUrl('/add-book');
      }
    });
  }

Service.ts

getBook(id) {
  console.log('called firebase get book');
  return this.db.doc(`/books/${id}`).valueChanges();
}

下面是Chrome控制台的图片。这表明订阅被调用了两次，但firebase服务没有被调用。
Chrome console logs: Image please click to view
Chrome控制台日志

called firebase get book
subscribe called
subscribe called

请帮