This simple 10 line code doesn't work if just the reference is changed..why?

Maybe like this (untested).

Sub Delete_Dup_Customer_Numbers()
    Dim iRow As Long
    
    With ActiveSheet.Columns("D")
        Do
            Do While WorksheetFunction.CountIf(.Cells, .Cells(iRow).Value) > 1
                .Find(.Cells(iRow).Value).EntireRow.Delete
            Loop
            iRow = iRow + 1
        Loop Until IsEmpty(.Cells(iRow).Value)
    End With
End Sub