I found that the kw2x is transmitting each 15.4 message twice with an interval of 2ms.

I first thought my sniffer has some problems, but then I started samr21 node which does not exhibit this behavior. But I saw this behavior with different kw2x nodes. I put a small wireshark capture in my dropbox here shows it.

Note: source addresses ending on 3016 and 600a are kw2x boards, the one with 47ee is the samr21. Though this capture only shows RA this also applies to other messages (and types), too.