In the project panel of the Web Editor, located on the left side of your screen, find your list of all sheets & scenes of the project. On the sheet that you would like to duplicate, right-click with your mouse.

You now have three options to choose from:
    1) Change name
    2) Duplicate (Sheet only)
    3) Duplicate with Location markers  

Click Duplicate (Sheet only). A copy of the sheet will now appear in your sheets & scenes list in the project panel. None of the location markers from the original sheet will be available on the new copy. It will be a "clean" sheet, ready to work with.