Validation on not required field - angularjs

Question

I have three dropdowns to set unique orders value.
Html Code:-
<form name="submitForm" ng-submit="save(submitForm.$valid)" novalidate="novalidate">
<div class="form-group">
<label>Unique order:</label>
<label>First Step</label>
<select ng-model="myOrder.step[0]" ng-change="checked()">
<option value="1">1</option>
<option value="2">2</option>
<option value="3">3</option>
</select>
<label>First Step</label>
<select ng-model="myOrder.step[1]" ng-change="checked()">
<option value="1">1</option>
<option value="2">2</option>
<option value="3">3</option>
</select>
<label>First Step</label>
<select ng-model="myOrder.step[2]" ng-change="checked()">
<option value="1">1</option>
<option value="2">2</option>
<option value="3">3</option>
</select>
</div>
<button type="submit" class="btn btn-success">Save</button>
</form>
On these dropdowns I applied a filter.
Directive Code:-
return {
restrict: 'EA',
scope: {},
controller: function ($scope) {
$scope.myOrder = {};
$scope.myOrder.step = [];
},
link: function (scope, el, attrs) {
scope.checked = function() {
if (_.uniq(scope.myOrder.step).length !== scope.myOrder.step.length) {
scope.notify("error", "Please set unique order value");
}
};
scope.save = function(isValid) {
if (isValid) {
if (!scope.values) {
ApiServices.updateOrderSet(scope.myOrder).then(function (response) {
scope.notify('success', response.data);
scope.values = false;
});
} else {
ApiServices.updateAddSet(scope.myOrder).then(function (response) {
scope.notify('success', response.data);
});
}
} else {
scope.notify('error', 'There are errors on the form');
}
};
},
What I want is if user set same values on dropdowns i.e., not unique ,then on save button it notifies scope.notify("error", "Please set unique order value"); also the form doesn't submit. Please note that these are not required fields.I tried $submitted too but doesn't work.Can anyone tell me what should I do here to make it work.