Prevent Duplicate Entries

Private Sub Worksheet_Change(ByVal Target As Range)
Dim r As Range
Dim ans As String
Const myCol As Long = 3
If Intersect(Target, Columns(myCol)) Is Nothing Then Exit Sub
Application.EnableEvents = False
For Each r In Intersect(Target, Columns(myCol))
If Application.CountIf(Columns(myCol), r.Value) > 1 Then
MsgBox (r.Value & " already exsists")
r.ClearContents
End If
Next
Application.EnableEvents = True
End Sub

Here the correct coding to solve duplicate entries in the same columns.