<label>1st</label>
  <select ng-model="a.value[0]">
      <option value="One">One</option>
      <option value="Two">Two</option>
      <option value="Three">Three</option>
  </select>
 <label>2nd</label>
 <select ng-model="a.value[1]">
      <option value="One">One</option>
      <option value="Two">Two</option>
      <option value="Three">Three</option>
 </select>
<label>3rd</label>
  <select ng-model="a.value[2]">
     <option value="One">One</option>
     <option value="Two">Two</option>
     <option value="Three">Three</option>
  </select>

$scope.a.value = [];
 scope.func = function () {
if (_.uniq(scope.a.value).length == scope.a.value.length) {
scope.notify("error", "Please set unique values");
 }
};