python/network/pcap_netmap: ignore repeated DEC MOP DNA_RC beacons
python/network/pcap_netmap: ignore repeated DEC MOP DNA_RC beacons

In corporate environments, it may happen that Cisco routers leak DEC MOP
frames: according to
https://networkengineering.stackexchange.com/questions/31/what-is-dec-mop-and-how-to-disable-it,
operators need to run "no mop enabled" on each interface of the routers.

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