CellEditDuplicate is an option for Cell which specifies whether the front end should make a copy of the cell before actually applying any changes in its contents that you request.
CellEditDuplicate is by default set to True for cells that are generated as Mathematica output.
New cells generated when CellEditDuplicate->True have styles specified by the setting for DefaultDuplicateCellStyle for the notebook.
CellEditDuplicate is typically set for styles of cells rather than for individual cells.
See The Mathematica Book: Section 2.10.9.
See also: CellEvaluationDuplicate, Editable.