Comment 31 for bug 657357

Still not fixed in Ubuntu 11.04 (at least for CLP-300 printer).
The solution is same: (may be helpful for new users)
1. system-config-printer
 set up your CLP-300 with default settings.
 make sure 600 dpi mode selected.
 close.
2. sudo gedit /etc/apt/sources.list
 add line
deb http://www.openprinting.org/download/printdriver/debian/ lsb3.2 main contrib main-nonfree
3. sudo apt-get update
4. sudo apt-get install openprinting-splix
5. system-config-printer
 Make and model -> Change
 select CLP-300 and you'll see two option at right pane, select SpliX.
Now duplex is work.