You can configure Duplie to automatically send an email when a volunteer signs up, cancels, and/or cancels (late).  Late cancellations are when a volunteer cancels within 24 (default) hours of their sign up.  You can setup different emails to notify based on event sign up role, and also across all sign ups.  Duplie will also send an in-app notification to all the admins when there is a late cancellation.