fix: deduplicate disks when calc disk capacity

[kxxt]

The disk capacity is wrong when a single file system is mounted multiple times, like:

/dev/mapper/root on /nix type btrfs (rw,relatime,compress=zstd:5,ssd,space_cache=v2,subvolid=263,subvol=/@nix)
/dev/mapper/root on /root type btrfs (rw,relatime,compress=zstd:5,ssd,space_cache=v2,subvolid=258,subvol=/@root)
/dev/mapper/root on /.snapshots type btrfs (rw,relatime,compress=zstd:5,ssd,space_cache=v2,subvolid=264,subvol=/@snapshots)
/dev/mapper/root on /home type btrfs (rw,relatime,compress=zstd:5,ssd,space_cache=v2,subvolid=257,subvol=/@home)
/dev/mapper/root on /srv type btrfs (rw,relatime,compress=zstd:5,ssd,space_cache=v2,subvolid=259,subvol=/@srv)
/dev/mapper/root on /var/cache type btrfs (rw,relatime,compress=zstd:5,ssd,space_cache=v2,subvolid=260,subvol=/@cache)
/dev/mapper/root on /var/lib/machines type btrfs (rw,relatime,compress=zstd:5,ssd,space_cache=v2,subvolid=267,subvol=/@machines)
/dev/mapper/root on /var/log type btrfs (rw,relatime,compress=zstd:5,ssd,space_cache=v2,subvolid=261,subvol=/@log)

It shows 16.15 TiB but the actual value should be near 2TB.

This PR fixes it by ignoring duplicated disks when calculating the disk capacity