Repeated editing of a non-Markdown post duplicates the line-breaks, which are br. Markdown uses p elements, which have no such problem.

Pumpa 0.9 on Debian

The problem is that editing a post has to be done in HTML. The non-Markdown mode doesn't handle this very well. The fix is to temporarily force Markdown mode when editing. Not elegant, but...
Comment by sazius [id.mayfirst.org] Wed Dec 24 12:25:47 2014