mount error
 

I have set one iso image to be mounted at boot in the fstab file.

However, at boot time, I get

mount:according to mtab /root/gparted.iso is already mounted or /mnt/iso busy

However, when I see after startup, iso image is mounted properly at /mnt/iso.

Are you sure that you don't have /mnt/iso referenced twice in your fstab file? The mtab file to which the error message refers just lists the mounted devices, and you're being told that the iso file was already listed as mounted. (The fact that you see the mount after you finish booting is, probably, a confirmation that the error has been correctly reported, and that (somehow) you've tried to mount the file twice.)

I have the entry once in the file.

If beating your head hurts, stop doing it. I've had similar issues on occasion, just add the mount to rc.local equivalent.

Could be /mnt is busy, could be loop devices aren't (completely) available at the time fstab is parsed, could be lots of things.