Duplicate accounting packets

I have a problem where accounting packets are not getting acknowledged by Freeradius.

I have been using the NTRadPing testing tool and have determined that if the Accounting Start packet contains a NAS-Port attribute, the packet will timeout.

If I remove the NAS-Port attribute from the Accounting Start packet I get an instant response.

Please see below output from radiusd -X - the line that seems to vary between successful and failed attempts is ++[unix] returns fail (its ++[unix] returns noop when successful).

Ready to process requests.
        User-Name = "80001@test.net.uk"
        Acct-Status-Type = Start
        Acct-Session-Id = "2476"
        NAS-IP-Address = 10.0.1.110
        NAS-Port = 1073
+- entering group preacct
++[preprocess] returns ok
rlm_acct_unique: Hashing 'NAS-Port = 1073,Client-IP-Address = 10.0.1.110,NAS-IP-Address = 10.0.1.110,Acct-Session-Id = "2476",User-Name = "80001@test.net.uk"'
rlm_acct_unique: Acct-Unique-Session-ID = "22a2bff7af67d3ea".
++[acct_unique] returns ok
    rlm_realm: Looking up realm "test.net.uk" for User-Name = "80001@test.net.uk"
    rlm_realm: Found realm "test.net.uk"
    rlm_realm: Proxying request from user 80001 to realm test.net.uk
    rlm_realm: Adding Realm = "test.net.uk"
    rlm_realm: Accounting realm is LOCAL.
++[suffix] returns noop
    acct_users: Matched entry DEFAULT at line 22
WARNING: Deprecated conditional expansion ":-". See "man unlang" for details
        expand: %{Stripped-User-Name:-%{User-Name}} -> 80001@test.net.uk
++[files] returns ok
+- entering group accounting
        expand: /var/log/radacct/%{Client-IP-Address}/detail-%Y%m%d -> /var/log/radacct/10.0.1.110/detail-20080502
rlm_detail: /var/log/radacct/%{Client-IP-Address}/detail-%Y%m%d expands to /var/log/radacct/10.0.1.110/detail-20080502
        expand: %t -> Fri May 2 09:06:38 2008
++[detail] returns ok
++[unix] returns fail

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