TLDR: it is not issue of Luban or other slicer, it is some bug directly in the firmware.
I don’t use Luban but Simplify3D and the printer is connected via USB to Raspberry Pi with Octoprint and have same issue - the duplication doesn’t work (only mirroring, see Can't use copy mode M605 S2 via back USB port - #7 by Viktor).
At the moment, the only working FW is 2.3.5 (but I am afraid is no more available for download) or you can compile the open source version from github. Last working commit is 1c21bcdcb12aa0223da74d55c7e59e52cbf6f4f0 (Pref: pause temp set timeout set to 2 miniute).
I have reported this issue both here and in the Github but no reaction from Snapmaker yet.