Literally can't name things properly
A CUPS backend/virtual printer that provides manual duplex (double-sided printing) functionality.
sudo make installpoppler-utils, zenity and perl.
Most distros have these packages installed out-of-the-box.
Add a printer in CUPS with the URI smdu://<Target Printer Name>, Use the Generic PDF Printer driver.
The "Target Printer" should be your real printing queue.
Or you can use the configure.pl script.
Print a multi-page document to test the functionality. If nothing happens, check /var/log/cups/error_log.
You can change the "Flip Paper" message by using a URI query parameter like ?flipMessage=Just+flip+then+over+and+you%27ll+be+fine.. The message should be urlencoded.
The backend script will be run with root permissions, so it's important patch security holes. All shell commands in the script should already be using the system() or open() function in order to prevent shell injection. More information in the blog post.
For more information, I have a blog post about this project.