Typically this 'duplicate authentication request id=xx already in progress' message means that we already got a authentication request or answer and that we are processing this (contacting the AAA server). If there is some delay there, the remote side will resend the authentication challenge/response and we will print out this message in the debug.
Try to figure out what the problem is with debug ppp negotiation, debug aaa authent, debug radius/tacacs, debug aaa author.
This should tell you where the delay is coming from (probably AAA server not responding)