behavior different from mdadm 2.6.4 and 3.0?

mdadm --incremental seems to behave differently for me between mdadm
2.6.4 and 3.0

With 2.6.4, mdadm --incremental /dev/nbd0 does this:

mdadm: /dev/nbd0 attached to /dev/md10 which is already active.

With 3.0, it does this:

mdadm: /dev/md10 is already in use.

and does *not* add it to the array.

-- 
Jon