title: PerformFetchWithCompletionHandler called twice when simulating with Xcode
body: <p>In Xcode 7.0.1 the "simulate background" fetch command causes <code>performFetchWithCompletionHandler</code> to be triggered twice. </p>
  
  <p>Is this an Xcode debugging error, or can this happen on a device running a release build of the application.</p>
  
  <p><strong>Update</strong>
  Now we have Xcode 7.1.1 and still <code>performFetchWithCompletionHandler</code> is called twice. Since I am not sure if this also happens "in the wild" I am keeping a state if my fetch action is already running.</p>
  
  <pre><code>- (void)application:(UIApplication *)application performFetchWithCompletionHandler:(nonnull void (^)(UIBackgroundFetchResult))completionHandler {
      if (self.performingFetch) {
          return completionHandler(UIBackgroundFetchResultNoData);
      }
      self.performingFetch = YES;
      ...
      self.performingFetch = NO;
  }
  </code></pre>