duplicated object when rendering i'm almost through a tutorial by " Andrew Price " and after i finished the modeling and texturing 

But when i rendered the Design i found out that the third small rail got duplicated i don't know why !
Look in the Outliner, you probably have an object whose visibility is disabled for the viewport (eye icon) but enabled for rendering (camera icon)



Or you have a duplicated object in a layer that is not visible on the 3D viewport but is renderable.