In my Solaris 10 based server, I have noticed the following mounts when a use DF -K

/dev/dsk/c0t0d0s0 5062414 3213876 1797914 65% /
/ 5062414 3213876 1797914 65% /net/se420

I understand the first mount because it appears in my vfstab file and is the mount of root that I would expect. However I do not understand the second mount as it appears to be a duplicate mount of the root partition. Also it does not appear in my vfstab. When I manually dismount this mount point and then reboot the server - it is remounted !

What is this second mount, where is it being auto mounted and do I need it anyway ?

It must be in your automounter maps somewhere. The master map is usually /etc/auto_master and it probably is there. If not, it will be in some file referenced by the master map. I usually just shut down automount.

Suppose you have a file in the root filesystem called /usr/somefile. But then you mount /usr, so you can no longer see /usr/somefile. A mount like you have there would restore access to that file. When /usr is mounted, the permissions on /usr are supposed to come from the root directory of the usr filesystem. Some, maybe all, versions of SunOS get the permissions instead from /usr in the root filesystem. Should they be wrong, you need to umount /usr, chmod /usr, and remount /usr. Or use this mount trick to gain access to the mount point. These are the only reasons I know to have a mount like that.