499 errors in IE9 causing order duplicates

[Paleo Tek]

We use nginx to reverse proxy to a farm of apache servers.  It rocks.  
But we've noticed that Internet Explorer 9 has been generating duplicate 
orders.  It does not happen every time, but often enough.  It appears to 
be an IE 9 issue, but I'm hoping the experts here can confirm my theory:

A request comes from an IE 9 client.  Nginx passes the request to an 
upstream apache server.  For some reason, the IE 9 browser closes the 
connection before the apache server returns results with return code 
200.  Nginx sees a closed connection, and generates a 499 error (client 
problem, connection closed).  IE 9 sees that, and immediately re-sends 
the request.  Nginx forwards to apache, receives the response, and 
forward to the client, logging a 200.  Log results are below

Does this seem correct?  Anyone else seeing this behavior?  Any 
suggestions for a graceful way to handle this?  (Duplicate orders BAD)  
Thanks,

          -Paleo

On nginx, we see:

XX.XX.XX.XX - - [14/Oct/2010:18:01:31 -0400] "POST /checkout.php  
HTTP/1.1" 499 0 "https://www.somesite.com/precheckout.php" "Mozilla/5.0 
(compatible; MSIE 9.0; Windows NT 6.1; Trident/5.0)" "-"

XX.XX.XX.XX - - [14/Oct/2010:18:01:33 -0400] "POST /checkout.php 
HTTP/1.1" 200 6996 "https://www.somesite.com/precheckout.php" 
"Mozilla/5.0 (compatible; MSIE 9.0; Windows NT 6.1; Trident/5.0)" "-"

And on apache, the same requests log:

XX.XX.XX.XX - - [14/Oct/2010:18:01:30 -0400] "POST /checkout.php 
HTTP/1.0" 200 6996 "https://www.somesite.com/precheckout.php" 
"Mozilla/5.0 (compatible; MSIE 9.0; Windows NT 6.1; Trident/5.0)" google

XX.XX.XX.XX - - [14/Oct/2010:18:01:30 -0400] "POST /checkout.php 
HTTP/1.0" 200 6996 "https://www.somesite.com/precheckout.php" 
"Mozilla/5.0 (compatible; MSIE 9.0; Windows NT 6.1; Trident/5.0)" google