duplex printing

[Anonymous]

How do I make a printer print duplex ?
I want to issue 'lp -dmy_duplex_laser my_document' but I want it to print duplex.  The laser printer has this capability, but I dont know how to activate this from the command line.
I have been able to 'hard' set this printer to print duplex all the time.  However, I want to be able to control this on a case by case basis.
Anyone know how to do this ?