I get the same behaviour on CubasePRO 8.5 and on 9.5:
I duplicate audio track (right click on original track).
When I try to edit duplicated track (select part, cut it move it…) both original track and duplicated parts on both tracks get selected simultaneously so I can not edit parts on both tracks individually.
It does happen only on my current project, some other older do not show this issue. I believe it must be something hidden or marked in preferences or settings.
Thank you for your help.