Package: ifupdown
Version: 0.8.35
Severity: normal

In /usr/share/man/man5/interfaces.5.gz the following example is giving for

       auto eth0
       allow-hotplug eth1

       iface eth0 inet dhcp

       iface eth0 inet6 auto

       iface eth1 inet static
            address 192.168.1.2/24
            gateway 192.168.1.1

       iface eth1 inet6 static
            address fec0:0:0:1::2/64
            gateway fec0:0:0:1::1

However using this example causes /lib/systemd/system/networking.service to 
fail with:

Jul  4 00:15:00 debian ifup[359]: RTNETLINK answers: File exists

because there are two "gateway" statements in /etc/network/interfaces

A possible  solution is to put the ipv6 part in /etc/network/interfaces.d/eth0

-- System Information:
Debian Release: 10.4
  APT prefers stable-updates
  APT policy: (500, 'stable-updates'), (500, 'stable')
Architecture: amd64 (x86_64)

Kernel: Linux 4.19.0-9-amd64 (SMP w/2 CPU cores)
Locale: LANG=en_US.UTF-8, LC_CTYPE=en_US.UTF-8 (charmap=UTF-8) (ignored: LC_ALL 
set to C.UTF-8), LANGUAGE=en_US:en (charmap=UTF-8) (ignored: LC_ALL set to 
C.UTF-8)
Shell: /bin/sh linked to /usr/bin/dash
Init: systemd (via /run/systemd/system)
LSM: AppArmor: enabled

Versions of packages ifupdown depends on:
ii  adduser   3.118
ii  iproute2  4.20.0-2
ii  libc6     2.28-10
ii  lsb-base  10.2019051400

Versions of packages ifupdown recommends:
ii  isc-dhcp-client [dhcp-client]  4.4.1-2

Versions of packages ifupdown suggests:
pn  ppp     <none>
pn  rdnssd  <none>

-- no debconf information

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