How to eliminate duplicate entries

I'm trying to find a way that when users enter a text that is already in a range, they get a msgbox telling them so.
I tried this method (data validation), but it doesn't work. Why?
It sounds simple, but I'm all out of ideas, please somebody help.

This is something I'm doing, but have no idea how to continue

Private Sub Worksheet_Change(ByVal Target As Range)

Dim Samples As Range
Dim Totaal As Range

Set Samples = Range("a:a")
Set Totaal = Range("a:b")

j = WorksheetFunction.CountA(Samples)

Set Samples = Range("a2:a" & j)

If WorksheetFunction.CountIf(Totaal, Samples) >= 2 Then
MsgBox "duplicate detected"
End If

End Sub

The error here is that Samples is a Range in stead of a Double. I don't know how to get around it. Anybody have an idea??