router receives both type 4 and type 1 DUID

Hi.

Checking the router logs, I see my desktop is sending both type 4 
(stable and independent from MAC) and type 1 (MAC) DUIDs.

I'm sure it's my desktop:
* I see them when I force NetworkManager to disconnect / reconnect
* the address corresponding to the type 4 is a static lease for this desktop
* ping6 to the address sent in response to the type 1 fails

Under IPV6, NM is configured with:
Connection method: automatic
Privacy Ext.: enabled, prefer temporary
Gen.mode: stable privacy

What gives? My laptop has an identical configuration and apparently only 
sends type 4 as I expected.

Also, the type 1 DUID is always used for "REBIND IA_NA" messages.

Thanks,
Andrea.