title: How to reapply validators to formControl having same name
body: <p>We have a form that have same formControlName for two fields. Only one field is visible at a time and validators are applied by below logic  - </p>
  
  <pre><code>  let validators: ValidatorFnUtil[] = [];
    validators.push(CustomValidators.validateNumberGreaterThanZero); //custom validators
     const control = this.accountForm.get('shares');
    control.setValidators(validators);
    control.updateValueAndValidity();
  </code></pre>
  
  <p>in template - </p>
  
  <pre><code>&lt;mat-form-field  &gt;
    &lt;mat-label&gt;{{ 'quantity' | translate }}&lt;/mat-label&gt;
    &lt;input
      type="number"
      formControlName="shares"
      matInput
    /&gt;
    &lt;mat-error *ngIf="formErrors.shares"&gt;{{
      formErrors.shares | translate
    }}&lt;/mat-error&gt;
  &lt;/mat-form-field&gt;
  </code></pre>
  
  <p>When form is loaded, we set the validator for the fields and it works as required.It verifies if the field is having negative value and highlights the field.The second field is visible when perform a slider and previous field gets hide.Now for this new field, validators don't work. If toggle back to previous state, it works for previous field.</p>