Rail duplication with pistons and slime blocks

Look at attached picture.
Then place minecart on rail.
The minecart activates the detector rail causing the piston to turn on, destroying the detector rail but dropping two instead of one.
If the slime is replaced with, for example stone, the rail is not duplicated, so it is probably only slime. - Supported by Fred (in comments)
Also happens if piston pushes from the side. - Fred
Only duplicates if piston is activated by said method (not redstone, buttons, levers or torches).

Happens with normal and sticky pistons.

(Happens in both Survival and Creative)