Duplicate Sheet on top of Sheet

I don't know how else to explain this. I thought it was just a screen glitch and shut everything down and restarted. I reopened my file and it's still there. I can also select from either the cells beneath or from the top most cells. This is a list I update, and now I'm not sure which cells have not been updated. Ideally, I'd want the top most layer of cells to stay. What did I do and how do I undo it?