More reports: two duplicate shares, both shortly after the devfee sessions (but not immediately), but four devfee sessions in total - so it doesn't happen for all devfee sessions. This is still better than before:
$ grep -P 'session|Acc|Rej' /log|grep -P 'session' -A 10
...
wolf [14:03:08] INFO - Start devfee session
wolf [14:03:43] INFO - Start user session
wolf [14:04:08] INFO - 17/0 Accepted : diff=2845616 : 70.19KH/s
wolf [14:04:30] INFO - 18/0 Accepted : diff=6288420 : 70.19KH/s
wolf [14:04:31] INFO - 19/0 Accepted : diff=8576415 : 70.19KH/s
wolf [14:05:19] INFO - 19/1 Rejected : diff=8553147, reason: Duplicate share2
wolf [14:05:58] INFO - 20/1 Accepted : diff=14668108 : 70.19KH/s
wolf [14:06:24] INFO - 21/1 Accepted : diff=2694462 : 70.19KH/s
wolf [14:06:57] INFO - 22/1 Accepted : diff=2706372 : 70.19KH/s
wolf [14:07:54] INFO - 23/1 Accepted : diff=4550410 : 70.20KH/s
wolf [14:08:32] INFO - 24/1 Accepted : diff=22443368 : 70.20KH/s
wolf [14:08:59] INFO - 25/1 Accepted : diff=15335047 : 70.20KH/s
--
wolf [15:01:09] INFO - Start devfee session
wolf [15:01:44] INFO - Start user session
wolf [15:01:50] INFO - 108/1 Accepted : diff=8045525 : 70.21KH/s
wolf [15:02:13] INFO - 109/1 Accepted : diff=3844769 : 70.21KH/s
wolf [15:04:02] INFO - 110/1 Accepted : diff=7749041 : 70.21KH/s
wolf [15:04:27] INFO - 111/1 Accepted : diff=70845897 : 70.21KH/s
wolf [15:05:58] INFO - 112/1 Accepted : diff=2787856 : 70.21KH/s
wolf [15:06:12] INFO - 113/1 Accepted : diff=3263759 : 70.21KH/s
wolf [15:06:18] INFO - 114/1 Accepted : diff=3381829 : 70.21KH/s
wolf [15:06:40] INFO - 115/1 Accepted : diff=6786242 : 70.21KH/s
wolf [15:07:13] INFO - 116/1 Accepted : diff=2733179 : 70.21KH/s
wolf [15:07:17] INFO - 117/1 Accepted : diff=13263367 : 70.21KH/s
--
wolf [15:59:11] INFO - Start devfee session
wolf [15:59:46] INFO - Start user session
wolf [15:59:46] INFO - 229/1 Accepted : diff=36833371 : 70.21KH/s
wolf [15:59:53] INFO - 229/2 Rejected : diff=71057445, reason: Duplicate share2
wolf [16:00:08] INFO - 230/2 Accepted : diff=30030473 : 70.21KH/s
wolf [16:00:18] INFO - 231/2 Accepted : diff=3413753 : 70.21KH/s
wolf [16:01:33] INFO - 232/2 Accepted : diff=5039366 : 70.21KH/s
wolf [16:01:59] INFO - 233/2 Accepted : diff=23368668 : 70.21KH/s
wolf [16:02:15] INFO - 234/2 Accepted : diff=4706655 : 70.21KH/s
wolf [16:02:55] INFO - 235/2 Accepted : diff=8716313 : 70.21KH/s
wolf [16:03:19] INFO - 236/2 Accepted : diff=2590195 : 70.21KH/s
wolf [16:04:19] INFO - 237/2 Accepted : diff=2623047 : 70.21KH/s
--
wolf [16:57:13] INFO - Start devfee session
wolf [16:57:48] INFO - Start user session
wolf [16:58:13] INFO - 347/2 Accepted : diff=13060685 : 70.12KH/s
wolf [16:58:19] INFO - 348/2 Accepted : diff=12609630 : 70.12KH/s
wolf [16:58:44] INFO - 349/2 Accepted : diff=3408431 : 70.12KH/s
wolf [16:58:48] INFO - 350/2 Accepted : diff=3239141 : 70.12KH/s
wolf [16:59:46] INFO - 351/2 Accepted : diff=2120311 : 70.12KH/s
wolf [17:00:00] INFO - 352/2 Accepted : diff=3220798 : 70.12KH/s
wolf [17:00:01] INFO - 353/2 Accepted : diff=1743769 : 70.12KH/s
wolf [17:00:17] INFO - 354/2 Accepted : diff=4832606 : 70.12KH/s
wolf [17:00:26] INFO - 355/2 Accepted : diff=2758301 : 70.12KH/s
wolf [17:00:29] INFO - 356/2 Accepted : diff=19267812 : 70.12KH/s