If I mount multiple block devices on the same mountpoint and try to unmount on of the block devices not on top, busybox unmount the top block device and return 0.

$ busybox mount /dev/sdb1 /mnt                                                                                                                                      
$ busybox mount /dev/sdb2 /mnt                                                                                                                                       
$ busybox mount | grep ^/dev/sdb                                                                                                                                     
/dev/sdb1 on /mnt type iso9660 (ro,relatime,nojoliet,check=s,map=n,blocksize=2048)
/dev/sdb2 on /mnt type vfat (rw,relatime,fmask=0022,dmask=0022,codepage=437,iocharset=ascii,shortname=mixed,errors=remount-ro)
$ busybox umount /dev/sdb1                                                                                                                                           
$ busybox mount | grep ^/dev/sdb                                                                                                                                     
/dev/sdb1 on /mnt type iso9660 (ro,relatime,nojoliet,check=s,map=n,blocksize=2048)