fix(scrolling): leaking subscription if same element is registered multiple times (#13974)

If the `ScrollDispatcher.register` method is called twice in a row, the consumer will end up with a subscription that can't be cleaned up. These changes add an extra check to prevent the same scrollable from being re-registered.

src/cdk/scrolling/scroll-dispatcher.spec.ts

@@ -112,6 +112,23 @@ describe('ScrollDispatcher', () => {
       subscription.unsubscribe();
     });
 
+    it('should not register the same scrollable twice', () => {
+      const scrollable = fixture.componentInstance.scrollable;
+      const scrollSpy = jasmine.createSpy('scroll spy');
+      const scrollSubscription = scroll.scrolled(0).subscribe(scrollSpy);
+
+      expect(scroll.scrollContainers.has(scrollable)).toBe(true);
+
+      scroll.register(scrollable);
+      scroll.deregister(scrollable);
+
+      dispatchFakeEvent(fixture.componentInstance.scrollingElement.nativeElement, 'scroll');
+      fixture.detectChanges();
+
+      expect(scrollSpy).not.toHaveBeenCalled();
+      scrollSubscription.unsubscribe();
+    });
+
   });
 
   describe('Nested scrollables', () => {

src/cdk/scrolling/scroll-dispatcher.ts

@@ -52,10 +52,10 @@ export class ScrollDispatcher implements OnDestroy {
    * @param scrollable Scrollable instance to be registered.
    */
   register(scrollable: CdkScrollable): void {
-    const scrollSubscription = scrollable.elementScrolled()
-        .subscribe(() => this._scrolled.next(scrollable));
-
-    this.scrollContainers.set(scrollable, scrollSubscription);
+    if (!this.scrollContainers.has(scrollable)) {
+      this.scrollContainers.set(scrollable, scrollable.elementScrolled()
+          .subscribe(() => this._scrolled.next(scrollable)));
+    }
   }
 
   /**