vba duplicates/empty cells -highlighting - excel

Question

I need to a create similar macro for duplicates (see below). The user will be able to choose a column symbol and then all the duplicates from the chosen column will be highlighted with color. I do not how to do that.
Below is the same idea but with empty cells.
Could you help? THX!
Sub EmptyCells()
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
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

Answer 1

Please make the following changes as per comments in your program. After changes it works for me. Assuming I have cleared existing formats on the worksheet and it is going to be first format condition.
Sub DuplicateCells() ' changed sub name
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
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)).Select ' Remove SpecialCells(xlCellTypeBlanks)
Selection.FormatConditions.AddUniqueValues 'Add this line
Selection.FormatConditions(Selection.FormatConditions.Count).SetFirstPriority 'Add this line
Selection.FormatConditions(1).DupeUnique = xlDuplicate 'Add this line
With Selection.FormatConditions(1).Interior ' add FormatConditions(1)
.Pattern = xlSolid
.PatternColorIndex = xlAutomatic
.Color = 65535
.TintAndShade = 0
.PatternTintAndShade = 0
End With
Selection.FormatConditions(1).StopIfTrue = False
End Sub
Results on my sample data are shown.