fix(select): don't emit change event multiple times when a reset option is selected twice in a row

Fixes the select's change events being fired when a reset option is selected twice in a row.

Fixes #10675.

Author: crisbeto

src/lib/select/select.spec.ts

@@ -2702,6 +2702,40 @@ describe('MatSelect', () => {
       expect(spy).toHaveBeenCalledWith('steak-0');
     }));
 
+    it('should not emit the change event multiple times when a reset option is ' +
+      'selected twice in a row', fakeAsync(() => {
+        const fixture = TestBed.createComponent(BasicSelectWithoutForms);
+        const instance = fixture.componentInstance;
+        const spy = jasmine.createSpy('change spy');
+
+        instance.foods[0].value = null;
+        fixture.detectChanges();
+
+        const subscription = instance.select.selectionChange.subscribe(spy);
+
+        fixture.debugElement.query(By.css('.mat-select-trigger')).nativeElement.click();
+        fixture.detectChanges();
+        flush();
+
+        (overlayContainerElement.querySelector('mat-option') as HTMLElement).click();
+        fixture.detectChanges();
+        flush();
+
+        expect(spy).toHaveBeenCalledTimes(1);
+
+        fixture.debugElement.query(By.css('.mat-select-trigger')).nativeElement.click();
+        fixture.detectChanges();
+        flush();
+
+        (overlayContainerElement.querySelector('mat-option') as HTMLElement).click();
+        fixture.detectChanges();
+        flush();
+
+        expect(spy).toHaveBeenCalledTimes(1);
+
+        subscription.unsubscribe();
+      }));
+
   });
 
   describe('with option centering disabled', () => {

src/lib/select/select.ts

@@ -877,11 +877,11 @@ export class MatSelect extends _MatSelectMixinBase implements AfterContentInit,
     } else {
       this._clearSelection(option.value == null ? undefined : option);
 
-      if (option.value == null) {
-        this._propagateChanges(option.value);
-      } else {
+      if (option.value != null) {
         this._selectionModel.select(option);
         this.stateChanges.next();
+      } else if (this.value !== option.value) {
+        this._propagateChanges(option.value);
       }
     }