Idempotent Send and Delivery Acknowledgements

A mobile client sends a message, but the network drops before it receives the "sent" acknowledgement. The client's retry logic resends the same message 2 seconds later.

1. Without any special handling, what goes wrong?
2. Design the client and server changes needed to make resends safe.
3. Extend your design to the sent/delivered/read acknowledgement chain — what happens if the "delivered" ack from the recipient's device is itself lost in transit?