Deliveries 8.3 for iOS Deliveries helps you keep track of all your packages, so you always know when they’re going to arrive….
Why am I getting duplicate events with the “Add to calendar” option?
If you sync your calendars and your deliveries between multiple devices, we recommend only turning on the “Add to calendar” option on one device. (Preferably the device you use the most often.) That way Deliveries can update the events on one device, and iCloud will take care of syncing them to your other devices.
If you're still wondering why this happens: let's say you add a delivery on your iPhone, and then Deliveries creates an event for it. Deliveries now syncs with your iPad… but your calendar hasn't quite synced yet. So an event is created on your iPad as well. Once calendar gets caught up, you now have two calendar events on both devices. If this does happen, Deliveries will eventually clear up the duplicates on its own. You can also manually delete duplicate events without causing any problems.