It seems that prometheus_sysctl_exporter(8) exports a few duplicate values.

Example line:

% prometheus_sysctl_exporter| fgrep sysctl_vfs_zfs_l2arc_write_max
sysctl_vfs_zfs_l2arc_write_max 8388608
sysctl_vfs_zfs_l2arc_write_max 8388608

Full error as written by node_exporter in /var/log/messages (a lot of times):

error gathering metrics: 11 error(s) occurred:\n*
[from Gatherer #2] collected metric \"sysctl_vfs_zfs_l2arc_write_max\" { untyped:<value:8.388608e+06 > } was collected before with the same name and label values\n*
[from Gatherer #2] collected metric \"sysctl_vfs_zfs_l2arc_write_boost\" { untyped:<value:8.388608e+06 > } was collected before with the same name and label values\n*
[from Gatherer #2] collected metric \"sysctl_vfs_zfs_l2arc_feed_min_ms\" { untyped:<value:200 > } was collected before with the same name and label values\n*
[from Gatherer #2] collected metric \"sysctl_vfs_zfs_l2arc_feed_secs\" { untyped:<value:1 > } was collected before with the same name and label values\n*
[from Gatherer #2] collected metric \"sysctl_vfs_zfs_l2arc_headroom\" { untyped:<value:2 > } was collected before with the same name and label values\n*
[from Gatherer #2] collected metric \"sysctl_vfs_zfs_arc_min\" { untyped:<value:0 > } was collected before with the same name and label values\n*
[from Gatherer #2] collected metric \"sysctl_vfs_zfs_l2arc_feed_again\" { untyped:<value:1 > } was collected before with the same name and label values\n*
[from Gatherer #2] collected metric \"sysctl_vm_uma_tcp_log_bucket_size\" { untyped:<value:30 > } was collected before with the same name and label values\n*
[from Gatherer #2] collected metric \"sysctl_vfs_zfs_arc_max\" { untyped:<value:0 > } was collected before with the same name and label values\n*
[from Gatherer #2] collected metric \"sysctl_vfs_zfs_l2arc_norw\" { untyped:<value:0 > } was collected before with the same name and label values\n*
[from Gatherer #2] collected metric \"sysctl_vfs_zfs_l2arc_noprefetch\" { untyped:<value:1 > } was collected before with the same name and label values

Right now I'm working around it like this:

% cat /usr/local/etc/cron.d/sysctl_exporter
*/1 * * * * root /usr/sbin/prometheus_sysctl_exporter | /usr/bin/sort -u > /var/tmp/node_exporter/sysctl_exporter.tmp ; mv /var/tmp/node_exporter/sysctl_exporter.tmp /var/tmp/node_exporter/sysctl_exporter.prom