Android - Why is there so many identical Android folders?

Because of how Android has evolved over the years many newer devices have the same files mounted (usually with a bind mount) in multiple places. Others are symbolic links. It's really too bad that the file manager app isn't showing you these details :-/

Prior to ICS, Android used /system/mnt/sdcard as the point for mount.

Some versions by manufacturers used /system/sdcard instead.

ICS, thereafter, changed it, but kept existing mount point for backwards compatibility with legacy apps.

Now, /system/storage/sdcard0 or /system/storage/sdcard1, particularly, JB on tablets, is used to accomodate multiple user accounts.

Crucial point is the name or alias for external storage is sdcard for legacy reasons and will stay that way. Or in technical parlance, symbolic link in filesystem terms points to sdcard.

Usually, 0 represented internal storage, 1 represented external storage.

The increased trend in dropping SDCard at expense of cheaper to manufacture rewriteable flash drives, bigger capacities, is prevalent.

I don't know about "legacy" or "0", but /sdcard is a symbolic link to the sdcard device folder.

At the terminal, run "ls -l". Add the end of some lines, you will see:

sdcard -> /mnt/sdcard
etc -> /system/etc

and maybe others depending on your setup. This is standard Linux setup, and makes both places link the same place - so there is no duplication.