I swear I've done this before and now I can't make it work.
You can't copy a sheet within a drawing to duplicate it. So, as a workaround, I saved a copy of the drawing to a different name and tried to copy a sheet back. Pick the view in the browser, right click, copy. Switch to the original drawing, right click paste in the browser. All I keep getting is an error "Invalid input for Request." I even think I remember getting this error before and finding a way around it.
I tried it yet again by saving the existing drawing to "temp", deleting all the views, copying the sheet and pasting it into the original drawing. I think there was at least a quit and restart of Inventor in there somewhere, too. This time it worked.