Look at the number of seconds between the first successful authentication, and the second failed reuse authentication. Steve is right about the re-transmission of the original authentication request, but if it is less than a second later, that could indicate a bug, or a problem with a High Availability pair of Agents 'accidentally' both sending the authentication the authentication. It could even be a loop in your network, though that is rare nowadays.
If the reuse authentication is from 2-5 seconds after the first authentication, that could be a simple configuration error, that did not allow enough time for the first request to be answered before it re-transmitted the same request. Most authentications need at least 5 seconds to fully complete, busy or oversubscribed AM servers might need 6 seconds.
There is a tcpdump utility in Linux on all AM servers, so you could capture all authentication traffic and compare the initial authentication with the reuse authentication, to compare details like source MAC or IP address, or IP layer ID and checksum to determine if the second request were caused by a layer 2 (Spanning Tree) loop or by a layer 3 routing or configuration issue.
