Thanks Noel for bringing this to upstream mailing list.

** The only feedback from coreutils is from Bernhard Voelker ***
<quote>

I'm not too enthusiastic for such an option.  This is the first time
I've seen someone doing "df *".  Although it doesn't seem to be explicitly
specified by POSIX, I'd assume a consistent behavior like many other tools
which do their job for each command line argument regardless whether it
may make sense (stat, du) or maybe not (df, dirname).

Furthermore, since the introduction of the output field 'file', it really
makes sense to not unify the output:


  $ df -h --out=source,target,file df du mv
  Filesystem     Mounted on      File
  /dev/sda5      /media/big_data df
  /dev/sda5      /media/big_data du
  /dev/sda5      /media/big_data mv

A --unique option would clash with this output format.

Finally, I'm not sure if everyone is aware that also regular files can
be a mount point (admittedly, the output of 'findmnt' is a little more
useful in this case):

  $ touch a b c

  $ mount --bind /proc/cpuinfo b
  $ mount --bind /sys/kernel/notes c

  $ df -h a b c
  Filesystem      Size  Used Avail Use% Mounted on
  /dev/sda2        20G   14G  5.2G  73% /
  proc               0     0     0    - /root/tmp/b
  sysfs              0     0     0    - /root/tmp/c

Still, when 2 lines are really identical (because they are on the
same file system and not mounted over), it may be valid to squeeze
the output, but I don't think this is following the principle of
least suprise: someone might be confused why some lines are missing
... or even relying on that in a script.  Anyway, one may simply
do that with "... | sort -u" if needed.

Have a nice day,
Berny
</quote>

Based on this, keeping opened to possibly adjust info documentation in some of the future updates. This is not something we can implement as downstream patch.

The output of "df *" is not non-intuitive.  It is just useless in most of the cases.  You are welcome to send a proposal to the bug-coreutils mailing list.