addons: bridge: don't warn several times for duplicate bridge-ports in stanza

addons: bridge: don't warn several times for duplicate bridge-ports in stanza

auto br0
iface br0
      bridge-ports swp1 swp2
      bridge-ports swp3 swp4

running ifquery in this configuration gaves us 2 identical warnings:

warning: br0: ignoring duplicate bridge-ports lines: ['swp3 swp4']
warning: br0: ignoring duplicate bridge-ports lines: ['swp3 swp4']

when running ifreload -a we still see 2 warnings, this will need to be looked
at later.

Signed-off-by: Julien Fortin <julien@cumulusnetworks.com>