basics: see #5603

#5603 is a special case (giving the same recursion root twice or more times) of a more generic case:

just imagine the same directory d (inode i) is mounted at multiple places a, b, ... in the filesystem.

if borg then backs up the filesystem starting at some recursion roots r, s, ..., it will run into an issue, if a, b, ... are contained in the directory tree below r, s, ... somehow.

borg will just come by the same inode i multiple times while doing the recursion.