I have observed the same problem on an up-to-date Jaunty machine: printing normal onesided, normal duplex or draft one-sided works fine, but the combination of draft grayscale and duplex consistently crashes. The relevant part of /var/log/cups/error_log is:

D [08/Aug/2009:20:28:09 +1200] PID 7033 (/usr/lib/cups/filter/pstopdf) exited with no errors.                                                 
D [08/Aug/2009:20:28:09 +1200] [Job 60] Filetype: PDF                                                                                         
D [08/Aug/2009:20:28:09 +1200] [Job 60] Storing temporary files in /var/spool/cups/tmp                                                        
D [08/Aug/2009:20:28:09 +1200] PID 7034 (/usr/lib/cups/filter/pdftopdf) exited with no errors.                                                
D [08/Aug/2009:20:28:09 +1200] [Job 60] File contains 4 pages                                                                                 
D [08/Aug/2009:20:28:09 +1200] [Job 60] Starting renderer with command: gs -dFirstPage=1  -q -dBATCH -dPARANOIDSAFER -dQUIET -dNOPAUSE -sDEVICE=ijs -sIjsServer=hpijs -dDEVICEWIDTHPOINTS=612 -dDEVICEHEIGHTPOINTS=792 -sDeviceManufacturer="HEWLETT-PACKARD" -sDeviceModel="deskjet 5600" -dDuplex=true -dTumble=false -r300 -sIjsParams=Quality:Quality=4,Quality:ColorMode=0,Quality:MediaType=0,Quality:PenSet=2,Quality:SpeedMech=1,PS:MediaPosition=7 -dIjsUseOutputFD -sOutputFile=-   /var/spool/cups/tmp/foomatic-8htL5x                                                          
D [08/Aug/2009:20:28:09 +1200] [Job 60] Starting process "kid3" (generation 1)                                                                
D [08/Aug/2009:20:28:09 +1200] [Job 60] Starting process "kid4" (generation 2)                                                                
D [08/Aug/2009:20:28:09 +1200] [Job 60] Starting process "renderer" (generation 2)                                                            
D [08/Aug/2009:20:28:09 +1200] [Job 60] JCL: 12345X@PJL                                                                                       
D [08/Aug/2009:20:28:09 +1200] [Job 60] <job data>                                                                                            
D [08/Aug/2009:20:28:09 +1200] [Job 60]                                                                                                       
I [08/Aug/2009:20:28:10 +1200] Saving subscriptions.conf...                                                                                   
D [08/Aug/2009:20:28:10 +1200] cupsdAcceptClient: 21 from localhost (Domain)                                                                  
D [08/Aug/2009:20:28:10 +1200] cupsdReadClient: 21 POST / HTTP/1.1                                                                            
D [08/Aug/2009:20:28:10 +1200] cupsdAuthorize: No authentication data provided.                                                               
D [08/Aug/2009:20:28:10 +1200] Get-Notifications /                                                                                            
D [08/Aug/2009:20:28:10 +1200] cupsdIsAuthorized: requesting-user-name="janet"                                                                
D [08/Aug/2009:20:28:10 +1200] cupsdProcessIPPRequest: 21 status_code=0 (successful-ok)                                                       
D [08/Aug/2009:20:28:10 +1200] cupsdCloseClient: 21                                                                                           
D [08/Aug/2009:20:28:11 +1200] [Job 60] Segmentation fault                                                                                    
D [08/Aug/2009:20:28:11 +1200] [Job 60] GPL Ghostscript 8.64: Unrecoverable error, exit code 1                                                
D [08/Aug/2009:20:28:11 +1200] [Job 60] renderer exited with status 1                                                                         
D [08/Aug/2009:20:28:11 +1200] [Job 60] Possible error on renderer command line or PostScript error. Check options.Kid3 exit status: 3        
E [08/Aug/2009:20:28:11 +1200] PID 7035 (/usr/lib/cups/filter/foomatic-rip) stopped with status 9!                                            
D [08/Aug/2009:20:28:26 +1200] PID 7036 (/usr/lib/cups/backend/hp) exited with no errors.                                                     
D [08/Aug/2009:20:28:26 +1200] [Job 60] File 0 is complete.                                                                                   
E [08/Aug/2009:20:28:26 +1200] [Job 60] Job stopped due to filter errors.                                                                     
I [08/Aug/2009:20:28:26 +1200] Saving subscriptions.conf...                                                                                   
I [08/Aug/2009:20:28:26 +1200] Saving subscriptions.conf...                                                                                    

And there is also a similar segfault error in dmesg (I neglected to copy it sorry, but I am pretty sure it is the same as that reported above).