fix(select): not marking options as selected correctly when setting value with duplicates

Fixes `mat-select` not marking all of the options as selected, when an array with duplicate values is assigned programmatically.

Fixes #13179.

Author: crisbeto

src/material/select/select.spec.ts

@@ -4329,6 +4329,32 @@ describe('MatSelect', () => {
       }).not.toThrow();
     }));
 
+    it('should be able to programmatically set an array with duplicate values', fakeAsync(() => {
+      testInstance.foods = [
+        { value: 'steak-0', viewValue: 'Steak' },
+        { value: 'pizza-1', viewValue: 'Pizza' },
+        { value: 'pizza-1', viewValue: 'Pizza' },
+        { value: 'pizza-1', viewValue: 'Pizza' },
+        { value: 'pizza-1', viewValue: 'Pizza' },
+        { value: 'pizza-1', viewValue: 'Pizza' },
+      ];
+      fixture.detectChanges();
+      testInstance.control.setValue(['steak-0', 'pizza-1', 'pizza-1', 'pizza-1']);
+      fixture.detectChanges();
+
+      trigger.click();
+      fixture.detectChanges();
+
+      const optionNodes = Array.from(overlayContainerElement.querySelectorAll('mat-option'));
+      const optionInstances = testInstance.options.toArray();
+
+      expect(optionNodes.map(node => node.classList.contains('mat-selected')))
+        .toEqual([true, true, true, true, false, false]);
+
+      expect(optionInstances.map(instance => instance.selected))
+        .toEqual([true, true, true, true, false, false]);
+    }));
+
   });
 });
 

src/material/select/select.ts

@@ -874,6 +874,12 @@ export class MatSelect extends _MatSelectMixinBase implements AfterContentInit,
    */
   private _selectValue(value: any): MatOption | undefined {
     const correspondingOption = this.options.find((option: MatOption) => {
+      // Skip options that are already in the model. This allows us to handle cases
+      // where the same primitive value is selected multiple times.
+      if (this._selectionModel.isSelected(option)) {
+        return false;
+      }
+
       try {
         // Treat null as a special reset value.
         return option.value != null && this._compareWith(option.value,  value);