Do you have a full license? Are you using CDF player? Are you able to add new cells? I'm not aware of any option called CellDuplicates. There is CellEditDuplicates and other similar, but they don't have anything to do with being able to edit notebooks.

You certainly can. You can select cells by clicking on the blue bracket on the right. You can also select groups. And you can use Shift-<select>, Command-<select> as you would in any other context. You can use Option-<select> to select all cells of the same type. There are even programmatic ways to select cells.