The following code will replace all the duplicate entries, except the first, in each group with the string "----".  to use this code, first select the cells (in one column) that you want to change, then run the procedure.

Sub FixDuplicaterows()
dim RowNdx As long
dim ColNum As Integer
ColNum = selection(1).Column
for RowNdx = selection(selection.cells.count).Row to _ 
        selection(1).Row + 1 Step -1
    if cells(RowNdx, ColNum).value = cells(RowNdx - 1, ColNum).value then
        cells(RowNdx, ColNum).value = "----"
    end if
next RowNdx
end Sub