Infinite rail duplication in 13w41b or (maybe) older

The first to third pictures show how the bug works and the duplication is confirmed in survival mode as shown in the last three pictures. It requires a placed rail first and creates two dropped rails. It works with powered rails, detector rails and activator rails but NOT normal rails.
The bug is only tested in 1.6.2 and 13w41b and is also confirmed in SMP(1.6.2 bukkit server). Players in older version report the bug works too but is not tested by me.
(Sorry for my poor English for I'm a high school student in China.)