2.1.3 is very old now , 2.1.9 is current and has many fixes over that - check its changelog .. this error message suggests that you've got a slow backend somewhere - be that ldap, sql or even a bit of perl

----- Reply message -----
From: "Mike Diggins" <mike.diggins@mcmaster.ca>
Date: Wed, Sep 15, 2010 16:22
Subject: Error: "Discarding duplicate request..."
To: "FreeRadius users mailing list" <freeradius-users@lists.freeradius.org>

Our students have returned this week, and I've noticed a couple new
messages logged to my FreeRadius 2.1.3 server. When it happens, my
controllers fail over to the secondary Radius server. This has happened
a few times. My Radius servers are only lightly loaded, and only
configured to do authentication. No databases. Any idea what might be
causing this?

Sep 15 10:06:44 prad02 radiusd[10632]: Discarding duplicate request from
client wlc-8 port 32769 - ID: 218 due to unfinished request 35236
Sep 15 10:07:01 prad02 radiusd[10632]: Discarding duplicate request from
client wlc-8 port 32769 - ID: 219 due to unfinished request 35237
Sep 15 10:07:24 prad02 radiusd[10632]: Discarding duplicate request from
client wlc-8 port 32769 - ID: 220 due to unfinished request 35239
Sep 15 10:07:41 prad02 radiusd[10632]: WARNING: Unresponsive child for
request 35239, in module  component
Sep 15 10:07:52 prad02 radiusd[10632]: WARNING: Unresponsive child for
request 35240, in module  component
Sep 15 10:07:53 prad02 radiusd[10632]: Discarding duplicate request from
client wlc-7 port 32769 - ID: 173 due to unfinished request 35240
Sep 15 10:07:53 prad02 radiusd[10632]: Discarding duplicate request from
client wlc-8 port 32769 - ID: 220 due to unfinished request 35239
Sep 15 10:07:53 prad02 radiusd[10632]: Discarding duplicate request from
client wlc-7 port 32769 - ID: 173 due to unfinished request 35240
Sep 15 10:07:53 prad02 radiusd[10632]: Discarding duplicate request from
client wlc-8 port 32769 - ID: 220 due to unfinished request 35239
Sep 15 10:07:53 prad02 radiusd[10632]: Discarding duplicate request from
client wlc-8 port 32769 - ID: 221 due to unfinished request 35244
Sep 15 10:07:53 prad02 radiusd[10632]: Discarding duplicate request from
client wlc-8 port 32769 - ID: 221 due to unfinished request 35244
Sep 15 10:07:53 prad02 radiusd[10632]: Discarding duplicate request from
client FHSWLC-1 port 32768 - ID: 205 due to unfinished request 35245

-Mike