Brian H thanks for the input.
"The phase coupler transmits X-10 commands that
enter from one phase to the other two phases. These
commands are repeated when transmitted to all phases."
(from the manual)
It is set to default, meaning repeating on all 3 phases and couple all 3 phases.
The funny thing is that the House Codes that do not cause a problem (B and C) use extension codes like dimming etc. and are mostly 2-way.
The house group G, causing the problem only uses ON/OFF with only one 2-way module.
Also it is not just an echo, since I also receive codes that do not exist on my system.
In Europe there are no (or not known to me) alternatives for the Phase coupler that I use.
A friend uses this one as wel in combination with the CM15PRO and has no issues.
Which does not mean that much since I have problem free House Codes B and C going as wel...