Duplicates - selected column - highlighting - colour

Hello,

I would like to create a macro where user will be able to choose the column symbol, then macro will colour all the duplicates in the column chosen.

I have created such macro but for empty cells. How could I change it for duplicates? Could you please help?

Below is the code for highlighting empty cells after users choice of the column symbol:

Sub empty()
Dim kol As String
Dim ost As Long
ost = Cells(Rows.Count, "A").End(xlUp).Row
 kol = InputBox("Enter column symbol: B, C...etc.", "Column symbol", "B")
 If kol = vbNullString Then
 Exit Sub
 End If
 If IsNumeric(kol) Then
 MsgBox "You entered number, please enter column symbol", vbInformation, "ERROR"
 Exit Sub
 End If
If ost < 5 Then Exit Sub
Range("A5:E" & ost).Interior.Color = xlNone
    Range(Cells(5, kol), Cells(ost, kol)).SpecialCells(xlCellTypeBlanks).Select
    With Selection.Interior
        .Pattern = xlSolid
        .PatternColorIndex = xlAutomatic
        .Color = 65535
        .TintAndShade = 0
        .PatternTintAndShade = 0
    End With
End Sub