This site has the following to say about the issue:
Two new RAID-related options appeared in the kernel options with Ubuntu 14.04 --
nomdmonisw. They appear to be underdocumented, and basically only relate to Ubuntu. AFAICT they relate to the plan to change from using
mdadm to assemble RAID devices; but I've been using mdadm for years.
The "nom nom nom" duplication seems to be caused by:
GRUB_CMDLINE_LINUX_DEFAULT="$GRUB_CMDLINE_LINUX_DEFAULT nomdmonddf nomdmonisw"
which unilaterally appends them to the config if the
mdadm package is installed, even if they are already there ( :-( ); and
dpkg-reconfigure grub-pc, which picks up those arguments and puts them into
/etc/default/grub, so that they're baked in for another round of adding them
update-grub also adding them onto the command line written into
/boot/grub/grub.cfg even if they're already there in the text coming from
There are some Ubuntu bugs #1291434 and #1318351 about this, so it might eventually be implemented better. Making the "ensure these parameters are present" idempotent -- so it only adds them once -- would seem like a good start!