On Debian network configuration can be done via /etc/network/interfaces.
It can accept a statement to load additional files, which varied in
history:
Wheezy only supports 'source' (see b822581)
Jessie supports 'source-directory' and comes with the statement by
default.
However since 754dd05 we inconditionally inject 'source', thus on Jessie
the configuration ends up with:
source-directory /etc/network/interfaces.d/*
source /etc/network/interfaces.d/*
When networking is started, 'ifup -a' parses the list of interfaces
twice. When configured with dhcp, that causes two dhclient to spawn
which might conflict with each other.
Inject the source statement only if there is neither a source or
source-directory with the same path.
Change-Id: Iefa9c9584f676e50481c621b4111eded3125a50b
2209d34372