events: don't call once twice

Emitting an event within a `EventEmitter#once` callback of the same
event name will cause subsequent `EventEmitter#once` listeners of the
same name to be called multiple times.

    var emitter = new EventEmitter();

    emitter.once('e', function() {
      emitter.emit('e');
      console.log(1);
    });

    emitter.once('e', function() {
      console.log(2);
    });

    emitter.emit('e');

    // Output
    // 2
    // 1
    // 2

Fix the issue, by calling the listener method only if it was not
already called.