Pop up message box if duplicate exist

Hi,
How can I pop up message box if duplicate exist in A column ?
I found following code but it is not working.
Please correct following code.
Thanks in advance.

Private Sub Worksheet_Change(ByVal Target As Range) 
    Dim Found As Range 
    If Target.Column <> 1 Then Exit Sub 
     
    Set Found = Range("A1", Cells(Rows.Count, 1).End(xlUp)).Find _ 
    (Target.Value, LookIn:=xlValues, lookat:=xlWhole) 
    If Not Found Is Nothing Then 
        If Target.Address = Found.Address Then Exit Sub 
        MsgBox "Ther is a duplicate entry at row " & Found.Row & "." 
        Target = "" 
    End If 
End Sub

Set Found = Range("A1", Cells(Rows.Count, 1).End(xlUp)).Find _ 
    (Target.Value, After:=Target.address, LookIn:=xlValues, lookat:=xlWhole)

Hi Leon V (AW),
I dont understand your answer.
Is it possible adding an excel file ?
Thank you.

Hi jindon,
I dont understand your answer.
Is it possible adding an excel file ?
Thank you.

Note: I dont want Private Sub Worksheet_Change(ByVal Target As Range) issue.
I want regular procedure...

I found following link regarding this issue.
Solution in following link is using conditional highlighting to show duplicates.
But I want to show duplicates in msgbox !
Any idea ?
http://www.excelforum.com/excel-prog...-column-a.html

Private Sub Worksheet_Change(ByVal Target As Range) 
    Dim Found As Range 
    If Target.Column <> 1 Then Exit Sub 
     
    Set Found = Range("A1", Cells(Rows.Count, 1).End(xlUp)).Find _ 
    (Target.Value, After:=Target.address, LookIn:=xlValues, lookat:=xlWhole)
    If Not Found Is Nothing Then 
        If Target.Address = Found.Address Then Exit Sub 
        MsgBox "Ther is a duplicate entry at row " & Found.Row & "." 
        Target = "" 
    End If 
End Sub

You could put Excel's vValidation on all the cells of column A with the formula
=(COUNTIF(A:A, A1)=1)

Solved...
http://www.mrexcel.com/forum/excel-q...cations-5.html