title: When registering for Java events, are duplicate listeners typically allowed?
body: <p>Should an <em>addListener</em> method check for duplicate registrations when called? If so, what should happen when a duplicate is found?</p>
  
  <pre><code>final public class exampleCanFire {
      public void addFooListener(FooListener listener) {
        // Before adding listener to private list of listeners, should I check for duplicates?
      }
  }
  </code></pre>