Let me try to explain what "mounting" really is for starters:
1. The data (files/directories) is stored on a physical hard drive, which you know and use via the special device files, called "sda2" and "sda3".
2. All the data from sda3 is mounted as your root partition. This means that the top level directory on sda3 will be shown to you as the "/" directory. Files and subdirectories on sda3 will thus also be shown as files/subdirectories under "/" as well, for instance /etc.
But how can you know use the data on your sda2 physical disk? You can't make the top level directory on sda2 appear as the "/" directory, because you already have said to your system that "/" is the top level dir of sda3 (you can only have one "/" dir).
So, to make the contents of sda2 visible in your global file hierarchy (directory structure), you will need to say to your system "put all the contents of disk sda2 and show it as directory x".
And this is exactly what mounting does. It takes the contents (files/dirs) of your sda2 drive and puts all that under /home, which is an empty directory on your sda3 disk. Think of it as "overlapping" sda2's contents on top of the /home (empty) on sda3.
However, mounting is done in a transparant way. So, when you are using your filesystem, ie accessing a file in /etc, /home, etc, the system will automatically figure out on which disk the file is really stored (ie /etc => sda3, /home/... => sda2) and it will locate the file for you.
So, the data is not "duplicated" in any way. It's simply distributed among multiple hard disks (partitions),
which are shown transparently as one big directory structure (going down from /).
To answer your backup question: it cannot "duplicate" the sda2 data, so don't worry.
If you tell tar to backup everything from / and down, it'll include the entire contents of /home (stored on sda2 if it is mounted) just once.
Note that, if you unmount sda2 via the "umount" command(not recommended, I'm just making a point), your /home will most be empty again, as it no longer represents the contents of sda2...
"sda2" is called the mounted partition, /home is called the "mount point" (the directory in your directory structure where the partition is mounted/shown).
and the /etc/mtab file (list of current mounts).
And yes, if your / partition becomes corrupted, your /home (sda2's contents) can still be there intact.