I believe something is wrong in CUPS/lp which is causing lp to print blank pages at inapropriate times for jobs with an odd page count.
Ticket #15 copied here:
This has broken all batch printing and I am seeking support on the issue. Even dplx/duplex is printing 2 even pages and then 1 odd page on a two page job that only requires one sheet - although it still works.
As a workaround, kmprb can still be used with a batch size of 1 which will force each job to be its own batch.... read more
Improved documentation and replaced duplex_0_1_2 today. No changes to code.
Version 0.1.2 was released today. It includes several new features and bug fixes. Notably, it fixes the syntax errors in the scripts caused by an update to bash since the previous release.