bind 9.11.0rc3 running as slave nameserver. Primary nameserver runs 9.10.4 P2.

On primary this happens (all that happens in 3 seconds):
- zone data is updated, serial set to 2016072731
- primary sends notify
- rndc notify test.zone.pl is called also (so second notify is send - this is unneded as first should be enough but that's how the system operates here currently)
- afer 2 seconds zone data is updated again, serial set to 2016072744
- primary sends notify
- rndc notify test.zone.pl is called also (so second notify is send)


How secondary server sees it:
29-Jul-2016 13:13:14.695 notify: info: client 1.1.1.1#30572: received notify for zone 'test.zone.pl'
29-Jul-2016 13:13:14.695 general: info: zone test.zone.pl/IN: notify from 1.1.1.1#30572: serial 2016072731: refresh in progress, refresh check queued
29-Jul-2016 13:13:19.695 notify: info: client 1.1.1.1#30572: received notify for zone 'test.zone.pl'
29-Jul-2016 13:13:19.695 general: info: zone test.zone.pl/IN: notify from 1.1.1.1#30572: serial 2016072731: refresh in progress, refresh check queued
29-Jul-2016 13:14:33.862 general: info: zone test.zone.pl/IN: Transfer started.
29-Jul-2016 13:14:33.868 xfer-in: info: transfer of 'test.zone.pl/IN' from 1.1.1.1#53: connected using 91.2.2.2#36328
29-Jul-2016 13:14:33.875 general: info: zone test.zone.pl/IN: transferred serial 2016072731
29-Jul-2016 13:14:33.875 xfer-in: info: transfer of 'test.zone.pl/IN' from 1.1.1.1#53: Transfer status: success
29-Jul-2016 13:14:33.875 xfer-in: info: transfer of 'test.zone.pl/IN' from 1.1.1.1#53: Transfer completed: 1 messages, 24 records, 1062 bytes, 0.007 secs (151714 bytes/sec)

so far good, zone with serial 2016072731 got transferred

29-Jul-2016 13:15:14.693 notify: info: client 1.1.1.1#64827: received notify for zone 'test.zone.pl'
29-Jul-2016 13:15:14.693 general: info: zone test.zone.pl/IN: notify from 1.1.1.1#64827: serial 2016072744: refresh in progress, refresh check queued

here we get first notify of zone with serial 2016072744 (so bigger than previously)

29-Jul-2016 13:15:19.693 notify: info: client 1.1.1.1#64827: received notify for zone 'test.zone.pl'
29-Jul-2016 13:15:19.693 general: info: zone test.zone.pl/IN: notify from 1.1.1.1#64827: serial 2016072744: refresh in progress, refresh check queued

here we got second notify (that one from rndc notify ...) with serial 2016072744

29-Jul-2016 13:16:08.208 general: info: zone test.zone.pl/IN: loaded serial 2016072731

yet bind happily ends process of loading zone with old serial  2016072731 and *no longer* refreshes zone with serial 2016072744. After that nothing
new happens with the zone and after about half hour we manually forced new notify via rndc notify.

In short words:

if notify with new serial is delivered to secondary ns while refresh process of the same zone with older serial is in progress then bind will
ignore new serial zone and will not fetch it again leaving secondary dns desynchronized from primary.

-- 
Arkadiusz Miśkiewicz, arekm / ( maven.pl | pld-linux.org )