In terms of connectivity, we deal with a similar problem here at CloudWalk to process payment transactions from POS terminals, where most of them rely on GPRS connections.
Our network issues are nearly 6 times higher (~3.5%) due to GPRS, and we solved the duplication problem with an approach involving both client and server side.
Clients would always ensure that all the information sent by the server was successfully received. If something goes wrong, instead of retrying (sending the payment again), the client sends just the transaction UUID to the server, and the server might either respond with: A. the corresponding response for the transaction or B. not found.
In the scenario A, the POS terminal managed to properly send all the information to the server but failed to receive the response.
In the scenario B, the POS terminal didn't even manage to properly send the information to the server, so the POS can safely retry.