SpreadsheetUI : Don't allow cells to be re-added to the same sheet
SpreadsheetUI : Don't allow cells to be re-added to the same sheet

johnhaddon/gaffer,lucienfostier/gaffer,johnhaddon/gaffer,ImageEngine/gaffer,johnhaddon/gaffer,GafferHQ/gaffer,boberfly/gaffer,lucienfostier/gaffer,appleseedhq/gaffer,johnhaddon/gaffer,andrewkaufman/gaffer,lucienfostier/gaffer,appleseedhq/gaffer,appleseedhq/gaffer,GafferHQ/gaffer,boberfly/gaffer,boberfly/gaffer,hradec/gaffer,hradec/gaffer,GafferHQ/gaffer,ImageEngine/gaffer,lucienfostier/gaffer,hradec/gaffer,hradec/gaffer,andrewkaufman/gaffer,johnhaddon/gaffer,ImageEngine/gaffer,GafferHQ/gaffer,andrewkaufman/gaffer,boberfly/gaffer,GafferHQ/gaffer,andrewkaufman/gaffer,ImageEngine/gaffer,hradec/gaffer,andrewkaufman/gaffer,appleseedhq/gaffer