We are implementing two-stage bootsrap here, where /etc is it's own FS which we first mount read-only and then need to promote to read-write later in the boot. There is an "update" option in fstab(5), which seemed like a solution for our need but there is also a logic in mount(8) which skips over already mounted filesystems. Which makes this option a no-op essentially. I believe if FS is marked with an "update" flag, the same logic applies as if -u command-line option has been specified.
$ ./mount_bug.sh + set -e + echo '/tmp /mnt nullfs ro 0 0' + echo '/tmp /mnt nullfs rw,update 0 0' + sudo mount -a -F fstab1 + sudo mount -a -F fstab2 + touch /mnt/somefile touch: /mnt/somefile: Read-only file system