Discarding Duplicate Request

I been seeing quite a large number of message like below logged in radius.log lately. Discarding duplicate request from client classroom98 port 32880 - ID: 131 due to unfinished request 241848 I read some thread, this might be the case when back-end server (i.e auth servers) are too slow to respond. My back-end is AD, using ntlm_auth. radius version 2.1.12-4 samba version 3.5.8-68 Any hints or suggestion how to resolve this would be very helpful. Most of the users get authenticated ( I don't think ntlm_auth is responding slow), I could not figure this out --cheers, Rando