This has happened most likely because you have selected the the extrude tool and then unselected it without using it , creating a duplicate of your mesh over the original.You have to be careful if you select the extrude tool and either use it to extrude or undo if you change your mind rather than just selecting another tool.
The bad news is there is no way to fix this apart from going over your model one polygon at a time and deleting the duplicate ...a hard lesson to learn but not forgotten im afraid .