This error is not really the cause of the problem, but it's telling you a problem exists.
What's happening is the VPN client sends a tunnel initialization packet to the 3005. the 3005 receives it, processes it and rpelies back to the client. The client, for some reason, doesn't receive this reply (this is the real problem here). Because the client never received a reply to it's first packet, it re-sends it. The 3005 receives this again, detects that it has already replied to this packet earlier, and writes a "duplicate packet" error in it's log.
If you open up the client log and then try and connect, you'll probably see the client sending packets out, not receiving any, and then re-transmitting the original packets.
So, you have to see why the packets from the 3005 to the client aren't getting through. Check you don't have a firewall of some sort either on the PC itself, or in between the 3005 and the client. You'll need to allow UDP port 500 through, or if you're doing IPsec/TCP then TCP port 10000 (default port).
Basically you have a connectivity problem, the "duplicate packet" error is merely telling you that packets are being retransmitted cause they're not getting through properly.
