An important note for users of the Samsung ML-4050ND laser printer

If you configure the paper type on the Samsung ML-4050ND to "Recycled," it thinks you mean that you are printing on the unused side of sheets that have already been printed on one side. Of course you don't want to print on the already-printed side, so it will helpfully disable the duplexer in this case, no matter what else you do. If you are printing on actual recycled paper, that is to say, paper made from recycled fibres, and you want duplex printing, then you must set your paper type to "Plain Paper." Of course, nothing in the Samsung, CUPS, Foomatic, or Ghostscript documentation, nor in the comments or content of any manufacturer or third-party PPD file (should you be crazy enough to hack PPD files manually), is remotely helpful in debugging this issue.