gunixmounts: Filter out mounts with device path that was repeated

libmount-based implementation doesn't filter out mounts with device
path that was repeated as it is done with mntent-based implementation.
It causes problems to our volume monitors which are not able to handle
multiple mounts for one device path properly without additional API.
Let's filter out the same mounts as are filtered out with mntent-based
implementation.

This is intended only for stable branches to prevent current issues.

#1271
9 jobs for <span class="ref-name">wip/oholy/bind-mounts-stable</span> in 9 minutes and 56 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Build
passed #127443
cross-android_api21_arm64

00:03:28

passed #127444
cross-android_api28_arm64

00:01:15

passed #127445
cross-mingw64

00:07:27

passed #127442
fedora-autotools-x86_64

00:06:55

passed #127441
fedora-x86_64

00:06:55

manual #127448
freebsd-11 allowed to fail manual
freebsd-11-x86_64
passed #127446
win32
msys2-mingw32

00:09:00

passed #127447
win32
vs2017-x64

00:05:22

 
  Coverage
passed #127449
coverage

00:00:56

73.5%