Here is image and it will tell everything. Left is rendered, right one is as you can see object mode shown as materials.
I am learning blender and this is my first project after watching one tutorial so i have never came up this problem.
It looks like you have more than one bench, occupying the same point in space. And this can destroy reality. Try to grab your bench with RMB in any direction. If there is another one, just kill it. Maybe it's hidden from the viewport, so first press Alt+H to reveal your hidden stuff.