title: not allow duplicates in a pureComputed element
body: <p>Having this element (note the computed field) <code>fullNumber</code></p>
  
  <pre><code>var NumberField = function () {
    var self = this;
    self.maskFormat = "0";
    self.firstNumber = ko.observable("");
    self.secondNumber = ko.observable("");
    self.thirdNumber = ko.observable("");
    self.fourthNumber = ko.observable("");
    self.fifthNumber = ko.observable("");
  
    self.fullNumber = ko.pureComputed(function() {
      return [
        self.firstNumber,
        self.secondNumber,
        self.thirdNumber,
        self.fourthNumber,
        self.fifthNumber ].map(ko.unwrap);
    }
  }
  </code></pre>
  
  <p>Then I use it like: </p>
  
  <pre><code>&lt;div class="numberFields" data-bind="with: numbers"&gt;
  
    &lt;input id="0" class="number-ball" maxlength="2" type="tel" data-bind="twoDigits: firstNumber, maxNumber: 68"&gt;
    &lt;input id="1" class="number-ball" maxlength="2" type="tel" data-bind="twoDigits: secondNumber, maxNumber: 68"&gt;
    &lt;input id="2" class="number-ball" maxlength="2" type="tel" data-bind="twoDigits: thirdNumber, maxNumber: 68"&gt;
    &lt;input id="3" class="number-ball" maxlength="2" type="tel" data-bind="twoDigits: fourthNumber, maxNumber: 68"&gt;
    &lt;input id="4" class="number-ball" maxlength="2" type="tel" data-bind="twoDigits: fifthNumber, maxNumber: 68"&gt;
  
  &lt;/div&gt;
  </code></pre>
  
  <p>How can I ensure there are no duplicated values in <code>fullNumber</code>?</p>