Comment 5 for bug 1908638

The behavior prior to the 14* patch was to only mount if (path, mountpoint) are not listed as mounted. But some mount helpers rewrite the path before mounting (like cryptsetup transitioning through /dev/mapper or NFS, as detailed in https://bugs.debian.org/799752). For those the check always fails and the mount would be redone on every login. The patch drops the path from the check as it is not predictable and only checks if mountpoint is listed as mounted.

Your use case is the other way round, you want to mount something on top of a mount point. The only way I see this works is by adding an option to not check for mounts at all. Maybe checkmounted?