Why `df` command doesn't show duplicated mounts?

Issue

• The df command does not display all active mount points when the same remote share (NFS/CIFS) or local block device is mounted multiple times to different directories.
• Running df -h or df -a fails to show all expected filesystems, even though they are confirmed to be active.
• The mount command or /proc/mounts correctly lists all mount points, but df only displays a subset (often only the first or last one mounted).
• At boot, all filesystems are mounted via /etc/fstab, but only one entry per source device appears in df.
• Manually unmounting and remounting a specific share may cause it to "appear" in df, potentially displacing the previous entry for that same source.