Why 2 dhclient processes running for eth0 (IPv4)?

[Clark Wang]

See following output from my system (Debian 8.6):

    root@debian:~# cat /etc/debian_version
    8.6
    root@debian:~# pgrep -af dhc
    506 dhclient -v -pf /run/dhclient.eth1.pid -lf
/var/lib/dhcp/dhclient.eth1.leases eth1
    447 dhclient -v -pf /run/dhclient.eth0.pid -lf
/var/lib/dhcp/dhclient.eth0.leases eth0
    601 dhclient -v -pf /run/dhclient.eth0.pid -lf
/var/lib/dhcp/dhclient.eth0.leases eth0
    664 dhclient -6 -pf /run/dhclient6.eth0.pid -lf
/var/lib/dhcp/dhclient6.eth0.leases eth0
    root@debian:~# grep -n -C3 eth0 /etc/network/interfaces
     7-auto lo
     8-iface lo inet loopback
     9-
    10:allow-hotplug eth0
    11:iface eth0 inet dhcp
    12:iface eth0 inet6 dhcp
    13-    accept_ra 1
    14-
    15-allow-hotplug eth1
    root@debian:~#

There are 2 `dhclient` processes running for `eth0` (IPv4). Rebooting does
not help for me.

Thanks.