Now the partitions are all gone from the sidebar. Only the 2 Windows partitions not mentioned in fstab are showing.
When I open Caja as admin I have the 3 double partitions showing once and being mounted (which should not be if I understand the fstab correctly).
A "sudo umount -a" makes them disappear as well (and some error about not removable devices.... lucky me linux is smarter than me here *g*)
A "mount -a" mounts my LMDE partition and nothing else (as it should be). The 3 partitions are now showing in the admin Caja ready to be mounted via click but not in Caja without admin rights. (Nemo/Nautilus same).
Here a screenshot ("Print" seems to be bugged, I got several different screenshots, first only my background then firefox, then only a few windows and finally the correct one (after a few Cinnamon restarts):
In the screenshot I unmounted TAUSCH. I finally know now what the "double" symbol was representing.