This is caused because you have 2 faces on top of one another, this is a common mistake by new users and what you most likely did was select the extrude tool and then deselected.
Once the extrude tool is selected the new faces are created imediately so you have to either make the extrude or hit ctrl + z to undo.

To correct this problem you have delete the faces on the model that are duplicated manually, but given the simplicy of the mesh it might be easier to remodel it being more careful when selecting the extrude tool.
Hope this is clear and helps.