Handle duplicate callbacks
Handle duplicate callbacks

- Fail gracefully on the callback being called twice

hasgeek/funnel,hasgeek/funnel,hasgeek/lastuser,hasgeek/lastuser,sindhus/lastuser,sindhus/lastuser,hasgeek/funnel,hasgeek/lastuser,hasgeek/lastuser,hasgeek/lastuser,sindhus/lastuser,sindhus/lastuser,hasgeek/funnel,hasgeek/funnel,sindhus/lastuser