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.
See #1271 (closed)