I reproduced the problem and verified that it is fixed by mdadm 3.2.5-5ubuntu4.2 from trusty-proposed. Additional "nomdmonddf nomdmonisw" entries are not appended when grub is reinstalled.
If you already had additional entries, they won't be removed when grub is reconfigured though.
I don't usually use mdadm, and I don't have a RAID array, so I can't test other than that.
I reproduced the problem and verified that it is fixed by mdadm 3.2.5-5ubuntu4.2 from trusty-proposed. Additional "nomdmonddf nomdmonisw" entries are not appended when grub is reinstalled.
If you already had additional entries, they won't be removed when grub is reconfigured though.
I don't usually use mdadm, and I don't have a RAID array, so I can't test other than that.