Since Linux 2.4.0 it is possible to remount part of the file hierarchy somewhere else. The call is
mount --bind olddir newdir
or fstab entry is:
/olddir /newdir none bind
After this call the same contents is accessible in two places. One can also remount a single file (on a single file).
This call attaches only (part of) a single filesystem, not possible submounts. The entire file hierarchy including submounts is attached a second place using
mount --rbind olddir newdir
Note that the filesystem mount options will remain the same as those on the original mount point, and cannot be changed by passing the -o option along with --bind/--rbind.