How is the provisioning done? Probably via the meshctl tool.
On the client node, only one element bound to a Generic OnOff Server model is subscribed to the group address?
Because I'm also running this Mesh onoff applic on 4 nRF52 DKs: 1 board for publishing, 2 boards for subscribing and 1 acting as relay node (for extending BLE coverage).
Further I modified the applic code a little bit so that it no longer waits for 1 second to publish when a button is pressed (no need in my use case to reuse the same button for switching on and off).
Honestly I didn't noticed a double receive in the logging. As I noticed an intial delay of 1 second I first focussed on getting rid off the logging, but the 1 second delay was caused by how the button press was implemented for supporting switching on/off by a single button.
If finding some time I will have a look at my logging when enabled again.