validar textbox vb.net

[julianrincon2005]

quiero que no me permita grabar nuevamente los registros sino que cuando aparezca uno que ya este en la base de datos me diga ya existe y pueda continuar
Gracias.........

Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
If SFANUMFAC.Text = "" Then
MsgBox("¡Tiene Que tener el campo NUMERO DE FACTURA ocupado!", vbExclamation, "Atencion")
Exit Sub
Else
Try
conexion.Open()
Dim daa As New SqlDataAdapter("VerFacturaRev", conexion)
daa.SelectCommand.CommandType = CommandType.StoredProcedure
daa.SelectCommand.Parameters.AddWithValue("@SFANUM FAC", Me.SFANUMFAC.Text)
'Dim dt As New DataTable
MsgBox("fakkkkkkkkkkkkkkkkkkctura ada")

If SFANUMFAC.Text <> "0" Then
'If SFANUMFAC.Text <> "@SFANUMFAC" Then

Dim cmd As New SqlCommand("InserFactReviz", conexion)
cmd.CommandType = CommandType.StoredProcedure
With cmd
.Parameters.AddWithValue("@SFANUMFAC", Me.SFANUMFAC.Text)
.Parameters.AddWithValue("@ADNINGRESO", Me.ADNINGRESO.Text)
.Parameters.AddWithValue("@SFAFECFAC", Me.SFAFECFAC.Text)
.Parameters.AddWithValue("@SFATOTFAC", Me.SFATOTFAC.Text)
.Parameters.AddWithValue("@SLNFACTUR", Me.SLNFACTUR.Text)
.Parameters.AddWithValue("@SFADOCANU", Me.SFADOCANU.Text)
.Parameters.AddWithValue("@GENUSUARIO1", Me.GENUSUARIO1.Text)
' .Parameters.AddWithValue("@imagen", Me.imagen)
.ExecuteNonQuery()
End With
MsgBox("factura agregada")

Else
MsgBox("¡Este Reguistro ya esta revizado!", vbExclamation, "Atencion")
End If

conexion.Close()
Catch ex As Exception
MsgBox(ex.Message)
End Try
End If
End Sub