Right click sheet tabcopy paste this. Now when you put something in cell b1
it will be added to the end of column A UNLESS it already exists in column A

Private Sub Worksheet_Change(ByVal Target As Range)
If Target.Address < Range("b1").Address Then Exit Sub
If Not Columns(1).Find(Target) Is Nothing Then Exit Sub
Cells(Cells(Rows.Count, 1).End(xlUp).Row + 1, 1) = Target
End Sub

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Don Guillett
Microsoft MVP Excel
SalesAid Software

"Dias" wrote in message
...
Hello!
I am looking for a formula that prevents the entry of duplicates.
Not insert a value in cell B1 when the value already exists in column A.
Any idea?
Thanks