for RAK3172 FW V4.1.1, in LoRaWAN mode.
For the first SEND comand it always get two time the +EVT:TX_DONE. For the next SENDs the answer is only one TX_DONE.
Is it expected like that?
As the RUI3 is open source now. Please, where is the latest code?
If there are downlinks queued in the LNS, it can happen that you see multiple TX Done messages. The device will get all of the queued downlinks. To do so, it well send empty uplinks.