duplicate ack in tcp closing state

Question for TCP/IP gurus.

Why fleetg sent duplicate ack at 11:01:56.033523? I have thousands
examples of this. That's annoying, as connections stuck in CLOSING
state. Could it be kernel (2.6.9-42.0.10.ELsmp) bug?

11:01:53.996951 IP client.3321 > fleetg.8090: S
1498869277:1498869277(0) win 5760 <mss 1380>
11:01:53.996976 IP fleetg.8090 > client.3321: S
4115165792:4115165792(0) ack 1498869278 win 5840 <mss 1460>
11:01:54.773653 IP client.3321 > fleetg.8090: . ack 1 win 5760
11:01:54.973885 IP client.3321 > fleetg.8090: . 1:43(42) ack 1 win
5760
11:01:54.973896 IP fleetg.8090 > client.3321: . ack 43 win 5840
11:01:55.161124 IP fleetg.8090 > client.3321: P 1:2(1) ack 43 win 5840
11:01:55.161145 IP fleetg.8090 > client.3321: F 2:2(0) ack 43 win 5840
11:01:55.813291 IP client.3321 > fleetg.8090: F 43:43(0) ack 2 win
5760
11:01:55.813307 IP fleetg.8090 > client.3321: . ack 44 win 5840
11:01:56.033510 IP client.3321 > fleetg.8090: . ack 3 win 5760
11:01:56.033523 IP fleetg.8090 > client.3321: . ack 44 win 5840
11:01:57.768679 IP fleetg.8090 > client.3321: F 2:2(0) ack 44 win 5840
11:02:01.679889 IP fleetg.8090 > client.3321: F 2:2(0) ack 44 win 5840
11:02:09.502842 IP fleetg.8090 > client.3321: F 2:2(0) ack 44 win 5840
11:02:25.148611 IP fleetg.8090 > client.3321: F 2:2(0) ack 44 win 5840
11:02:56.439377 IP fleetg.8090 > client.3321: F 2:2(0) ack 44 win 5840
11:03:59.021819 IP fleetg.8090 > client.3321: F 2:2(0) ack 44 win 5840
11:03:59.045316 IP client.3321 > fleetg.8090: R
1498869321:1498869321(0) win 0

Thank you
--fedya