I suspect I have multiple Ubuntu installations on different partitions on my PC. But how do I find out about them? Which command to use?

Output of

 sudo lsblk -o NAME,FSTYPE,SIZE,MOUNTPOINT,LABEL

is

NAME FSTYPE SIZE MOUNTPOINT LABEL
sda 931.5G 
├─sda1 ext4 660G /home/sda1_mountpoint 709G
├─sda2 1K 
├─sda5 swap 7.7G [SWAP] 
├─sda6 vfat 123.4G /data124G 132G
├─sda7 ext4 74.3G /data73G 80G
└─sda8 ext4 66.1G / 
sdb ext4 931.5G 
└─sdb1 ext4 931.5G /data1T data1T
sr0 1024M

and Output of

sudo parted -l

is

Model: ATA TOSHIBA DT01ACA1 (scsi)
Disk /dev/sda: 1000GB
Sector size (logical/physical): 512B/4096B
Partition Table: msdos
Disk Flags: 

Number Start End Size Type File system Flags
 1 32.3kB 709GB 709GB primary ext4
 2 709GB 1000GB 291GB extended
 7 709GB 788GB 79.7GB logical ext4
 6 788GB 921GB 132GB logical fat32
 8 921GB 992GB 71.0GB logical ext4
 5 992GB 1000GB 8266MB logical linux-swap(v1)


Model: ATA ST1000DM003-1ER1 (scsi)
Disk /dev/sdb: 1000GB
Sector size (logical/physical): 512B/4096B
Partition Table: msdos
Disk Flags: 

Number Start End Size Type File system Flags
 1 1049kB 1000GB 1000GB primary ext4

Background info - Some problem occurred when PC was not restarting. It was stuck in login loop and similar error to this was occuring. I did ctrl-alt-f1 . I found that user's home directory was missing i.e/ /home/ was empty! I did mkdir /home/user and copied an old backuped home directory there and restarted. It started fine. Later I mounted sda1 at /home/sda1_mountpoint. It gave missing superblock error. I had to do fsck, then it mounted. the missing home directory was found in sda1 .

So I think problem is with multiple installations. Also notice that none of the output of lsblk is /home.

EDIT After reading comments, I mounted sda1 to /home instead of /home/sda1_mountpoint

Output of etc/fstab is

# /etc/fstab: static file system information.
#
# Use 'blkid' to print the universally unique identifier for a
# device; this may be used with UUID= as a more robust way to name devices
# that works even if disks are added and removed. See fstab(5).
#
# <file system> <mount point> <type> <options> <dump> <pass>
# / was on /dev/sda8 during installation
UUID=10138724-bb55-4d41-b8f8-81fc42ec1a84 / ext4 errors=remount-ro 0 1
# swap was on /dev/sda5 during installation
UUID=7d10a86a-3139-45af-bfc5-3dbb26a3e767 none swap sw 0 0
# sda1
UUID=b50f72de-3b8b-4453-a743-cc37f06055a5 /home ext4 defaults 0 2 
# sda6
UUID=DBA5-EDA5 /data124G vfat defaults 0 0 
# sda7
UUID=01e6d118-230d-4f22-a528-3e7ff06aef39 /data73G ext4 defaults 0 0
# sdb1
UUID=a92ea8c4-c810-4c06-8586-285bed30ea3f /data1T ext4 defaults 0 0 

Output of mount command is

sysfs on /sys type sysfs (rw,nosuid,nodev,noexec,relatime)
proc on /proc type proc (rw,nosuid,nodev,noexec,relatime)
udev on /dev type devtmpfs (rw,nosuid,relatime,size=12301796k,nr_inodes=3075449,mode=755)
devpts on /dev/pts type devpts (rw,nosuid,noexec,relatime,gid=5,mode=620,ptmxmode=000)
tmpfs on /run type tmpfs (rw,nosuid,noexec,relatime,size=2464372k,mode=755)
/dev/sda8 on / type ext4 (rw,relatime,errors=remount-ro,data=ordered)
securityfs on /sys/kernel/security type securityfs (rw,nosuid,nodev,noexec,relatime)
tmpfs on /dev/shm type tmpfs (rw,nosuid,nodev)
tmpfs on /run/lock type tmpfs (rw,nosuid,nodev,noexec,relatime,size=5120k)
tmpfs on /sys/fs/cgroup type tmpfs (ro,nosuid,nodev,noexec,mode=755)
cgroup on /sys/fs/cgroup/systemd type cgroup (rw,nosuid,nodev,noexec,relatime,xattr,release_agent=/lib/systemd/systemd-cgroups-agent,name=systemd)
pstore on /sys/fs/pstore type pstore (rw,nosuid,nodev,noexec,relatime)
cgroup on /sys/fs/cgroup/net_cls,net_prio type cgroup (rw,nosuid,nodev,noexec,relatime,net_cls,net_prio)
cgroup on /sys/fs/cgroup/pids type cgroup (rw,nosuid,nodev,noexec,relatime,pids)
cgroup on /sys/fs/cgroup/memory type cgroup (rw,nosuid,nodev,noexec,relatime,memory)
cgroup on /sys/fs/cgroup/blkio type cgroup (rw,nosuid,nodev,noexec,relatime,blkio)
cgroup on /sys/fs/cgroup/hugetlb type cgroup (rw,nosuid,nodev,noexec,relatime,hugetlb)
cgroup on /sys/fs/cgroup/perf_event type cgroup (rw,nosuid,nodev,noexec,relatime,perf_event)
cgroup on /sys/fs/cgroup/cpu,cpuacct type cgroup (rw,nosuid,nodev,noexec,relatime,cpu,cpuacct)
cgroup on /sys/fs/cgroup/cpuset type cgroup (rw,nosuid,nodev,noexec,relatime,cpuset)
cgroup on /sys/fs/cgroup/devices type cgroup (rw,nosuid,nodev,noexec,relatime,devices)
cgroup on /sys/fs/cgroup/freezer type cgroup (rw,nosuid,nodev,noexec,relatime,freezer)
systemd-1 on /proc/sys/fs/binfmt_misc type autofs (rw,relatime,fd=26,pgrp=1,timeout=0,minproto=5,maxproto=5,direct)
mqueue on /dev/mqueue type mqueue (rw,relatime)
debugfs on /sys/kernel/debug type debugfs (rw,relatime)
hugetlbfs on /dev/hugepages type hugetlbfs (rw,relatime)
fusectl on /sys/fs/fuse/connections type fusectl (rw,relatime)
tracefs on /sys/kernel/debug/tracing type tracefs (rw,relatime)
/dev/sdb1 on /data1T type ext4 (rw,relatime,data=ordered)
/dev/sda6 on /data124G type vfat (rw,relatime,fmask=0022,dmask=0022,codepage=437,iocharset=iso8859-1,shortname=mixed,errors=remount-ro)
/dev/sda7 on /data73G type ext4 (rw,relatime,data=ordered)
/dev/sda1 on /home type ext4 (rw,relatime,data=ordered)
tmpfs on /run/user/108 type tmpfs (rw,nosuid,nodev,relatime,size=2464372k,mode=700,uid=108,gid=114)
gvfsd-fuse on /run/user/108/gvfs type fuse.gvfsd-fuse (rw,nosuid,nodev,relatime,user_id=108,group_id=114)
tmpfs on /run/user/1000 type tmpfs (rw,nosuid,nodev,relatime,size=2464372k,mode=700,uid=1000,gid=1000)

There is no sign that you have two Ubuntu installations on your hard drive. If there were multiple Linux installation and you had all meaningful partitions mounted (as you indeed have according to the lsblk output), you would get duplicate results if you tried to find essential system files like shadow (normally located in /etc/).

find / -type f -name shadow 2> /dev/null

Restoring the original /home contents

The only thing you may have duplicated is the home folder:

I found that user's home directory was missing i.e/ /home/ was empty! I did mkdir /home/user and copied an old backuped home directory there and restarted.

However, you seem to have discovered the correct solution already:

Later I mounted sda1 at /home/sda1_mountpoint. It gave missing superblock error. I had to do fsck, then it mounted. the missing home directory was found in sda1.

Since the mount is still present in your /etc/fstab,

# sda1
UUID=b50f72de-3b8b-4453-a743-cc37f06055a5 /home ext4 defaults 0 2

it should mount automatically on next reboot. If not, the UUID has probably changed, so you should update it in this file. You can get the current UUID using blkid:

sudo blkid /dev/sda1

Removing the duplicated /home contents

The only remaining task is to get rid of the duplicate home folder contents. The contents of /home on the root partition are still present on the disk, although it’s overlaid using mount. This is not necessary but the data are still occupying your disk space… To access them, mount the original contents elsewhere, e.g. to /mnt:

sudo mount --bind /home /mnt

Then open the /mnt directory, you should see the contents of your temporary /home there. Delete or move them and unmount it:

sudo umount /mnt

References

• https://unix.stackexchange.com/questions/658/linux-how-can-i-view-all-uuids-for-all-available-disks-on-my-system


• https://unix.stackexchange.com/questions/4426/access-to-original-contents-of-mount-point


• https://unix.stackexchange.com/questions/251090/why-does-mount-happen-over-an-existing-directory