[plt-scheme] Multiple network sockets on the same port?

The error:
..\actinic\actinic.ss::15268: tcp-connect: connection to 10.0.5.5, port 
80 failed (at step 6: Only one usage of each socket address 
(protocol/network address/port) is normally permitted.; errno=10048)

I am writing web crawlers and file transfer agents using lots of 
threads.  This error occurs when I try to shift a file using multiple 
threads (to reduce latency).

I'm not quite sure what that error means.  If there has been a collision 
of source ports, then something has gone very wrong in network layer.  
But I think it means that I can't open two connections to the same port 
on the same server.

If the latter, would it be possible to have that restriction lifted or 
is it too fundamental to change?  It's very common to open multiple 
connections to a server when doing web work.

yours,
Jeremy