CellEvaluationDuplicate is an option for Cell which specifies whether the front end should make a copy of the cell before performing any evaluation of its contents that you request.
New cells generated when CellEvaluationDuplicate->True have styles specified by the setting for DefaultDuplicateCellStyle for the notebook.
CellEvaluationDuplicate is typically set for styles of cells rather than for individual cells.
See The Mathematica Book: Section 2.10.9.
See also: CellEditDuplicate, Evaluatable, CellAutoOverwrite.