title: how to restrict duplicate values from dropdown in angular2
body: <p>Here i have mentioned current image form.
  <img src="https://i.stack.imgur.com/cPg5D.png" alt="image"></p>
  
  <p>1) It is an multiple select dropdown. it should restrict duplicate tax name.
  for an example in this image two time that <strong>CGST</strong> selected. </p>
  
  <p>while selecting same name it should show not display here.
  so please help me to do this. </p>
  
  <p><strong>html</strong></p>
  
  <pre class="lang-html prettyprint-override"><code>&lt;div class="form-group"&gt;
    &lt;label for="tax"&gt;Tax Name&lt;/label&gt;
    &lt;select class="form-control" id="tax_name" (change)=" dropdown(tax.value)" [(ngModel)]="model.tax" name="tax_name" #tax="ngModel"&gt;
      &lt;option value="addNew"&gt;
        &lt;i class="fa fa-plus" aria-hidden="true"&gt;&lt;/i&gt;Add New Tax &lt;/option&gt;
      &lt;hr&gt;
      &lt;br/&gt;
      &lt;option *ngFor="let i of taxlist" [value]="i.tax_name"&gt;{{i.tax_name}} &amp;nbsp; ( {{i.tax_percentage}}% )&lt;/option&gt;
    &lt;/select&gt;
  &lt;/div&gt;
  &lt;div&gt;
  
    &lt;label for="percentage"&gt;Tax Percentage&lt;/label&gt;
    &lt;input type="text" class="form-control" id="tax_percentage" placeholder="Percentage" pattern="[0-9]+" minlength="0" maxlength="3"
      [(ngModel)]="per" name="tax_percentage" #percentage="ngModel"&gt;
  &lt;/div&gt;
  &lt;button (click)="addContact(tax.value,percentage.value);" type="button" class="btn btn-success btn-sm text-right"&gt;
    &lt;i class="fa fa-plus fa-lg"&gt;&lt;/i&gt;
  &lt;/button&gt;
  </code></pre>