To copy an entire Google spreadsheet, just open the spreadsheet and choose File > Make a copy.... A copy of this spreadsheet will appear in your Documents list.
To copy one sheet within a spreadsheet, follow these steps:
- On the bottom-left side of your spreadsheet, click the tab of the sheet you'd like to copy (i.e. "Sheet1").
- Select Duplicate from the pop-up menu.
A duplicate sheet (which contains the exact formatting) now appears in a tab beside the original shee. It'll be labeled Copy of Sheet1.