Question: duplicated instance when /etc/crontab set to run hourly at 00 minute

If I set /etc/cron.hourIy/btrbk to run at 00 min each hour, btrbk reports an error through email with the following error every midnight.

It won't have a problem if I run it at other schedule such as min 22 each hour.

The email looks like the following:
> Subject: btrbk <$hostname> ERROR - Another instance of btrbk is running, no backup tasks performed!
> Content: Running btrbk: ERROR (code=3): Another instance of btrbk is running, no backup tasks performed!
>
>
> DETAIL:
>
> ### Running btrbk
> ++ 'btrbk' '-c' '/etc/btrbk/btrbk.conf' 'run'
> . ERROR: Failed to take lock (another btrbk instance is running): /var/lock/btrbk.lock
>
> ### ERROR (code=3): Another instance of btrbk is running, no backup tasks performed!

The content of the file /etc/btrbk/btrbk.conf:
```
#
# btrbk configuration file
#
#
# Please refer to the btrbk.conf(5) man-page for a complete
# description of all configuration options.
# For more examples, see README.md included with this package.
#
# btrbk.conf(5):
# README.md:
#
# Note that the options can be overridden per volume/subvolume/target
# in the corresponding sections.
#

# Enable transaction log
transaction_log /var/log/btrbk.log

# timestamp
timestamp_format long

# Specify SSH private key for remote connections
ssh_identity /etc/btrbk/ssh/id_ecdsa
ssh_user nuc13

# Use sudo if btrbk or lsbtr is run by regular user
#backend_local_user btrfs-progs-sudo
backend_remote btrfs-progs-sudo

######################################################################################################
# Ignoring errors on the remote system.
# It does not guarantee the backups can be restored but we don't have a choice when backing up to DSM7
######################################################################################################
compat_remote ignore_receive_errors

# Enable stream buffer. Adding a buffer between the sending and
# receiving side is generally a good idea.
# NOTE: If enabled, make sure to install the "mbuffer" package!
stream_buffer 256m

# Directory in which the btrfs snapshots are created. Relative to
# of the volume section.
# If not set, the snapshots are created in .
#
# If you want to set a custom name for the snapshot (and backups),
# use the "snapshot_name" option within the subvolume section.
#
# NOTE: btrbk does not automatically create this directory, and the
# snapshot creation will fail if it is not present.
#
snapshot_dir _btrbk_snapshots

# Always create snapshots. Set this to "ondemand" to only create
# snapshots if the target volume is reachable. Set this to "no" if
# snapshot creation is done by another instance of btrbk.
snapshot_create always

# Perform incremental backups (set to "strict" if you want to prevent
# creation of non-incremental backups if no parent is found).
incremental yes

# Specify after what time (in full hours after midnight) backups/
# snapshots are considered as a daily backup/snapshot
preserve_hour_of_day 0

# Specify on which day of week weekly/monthly backups are to be
# preserved.
preserve_day_of_week sunday

# Preserve all snapshots for a minimum period of time.
snapshot_preserve_min 48h

# Retention policy for the source snapshots.
#snapshot_preserve h d w m y
snapshot_preserve 72h 14d 10w

# Preserve all backup targets for a minimum period of time.
target_preserve_min no

# Retention policy for backup targets:
#target_preserve h d w m y
target_preserve 14d 10w

# Retention policy for archives ("btrbk archive" command):
#archive_preserve_min no
#archive_preserve h d w m y

# Enable compression for remote btrfs send/receive operations:
#stream_compress no
#stream_compress_level default
#stream_compress_threads default

# Enable lock file support: Ensures that only one instance of btrbk
# can be run at a time.
lockfile /var/lock/btrbk.lock

# Don't wait for transaction commit on deletion. Enable this to make
# sure the deletion of subvolumes is committed to disk when btrbk
# terminates.
#btrfs_commit_delete no

#
# Volume section (optional): "volume "
#
# Base path within a btrfs filesystem
# containing the subvolumes to be backuped
# (usually the mount-point of a btrfs filesystem
# mounted with subvolid=5 option).
#
# Subvolume section: "subvolume "
#
# Subvolume to be backuped, relative to
# in volume section.
#
# Target section: "target "
#
# (optional) type, defaults to "send-receive".
# Directory within a btrfs filesystem
# receiving the backups.
#
# NOTE: The parser does not care about indentation, this is only for
# human readability. All options apply to the last section
# encountered, overriding the corresponding option of the upper
# section. This means that the global options must be set on top,
# before any "volume", "subvolume" or "target section.
#

volume /mnt/btr_pool
subvolume rootfs
snapshot_dir _btrbk_snapshots/rootfs
target ssh://10.27.0.40/volume2/nuc13_backups/rootfs

subvolume docker
snapshot_dir _btrbk_snapshots/docker
target ssh://10.27.0.40/volume2/nuc13_backups/docker

# subvolume docker/nextcloud/data
# snapshot_dir _btrbk_snapshots/nextcloud_data
# snapshot_name nextcloud_data
# target ssh://10.27.0.40/volume2/nuc13_backups/nextcloud/data

subvolume docker/nc/mariadb/config/databases
snapshot_dir _btrbk_snapshots/nextcloud_mariadb_databases
snapshot_name nextcloud_mariadb_databases
target ssh://10.27.0.40/volume2/nuc13_backups/nextcloud/mariadb_databases
```