Evitar registros duplicados en la BD

[biblio]

Checa esto.
Set rsCheckUser = Server.CreateObject("ADODB.Recordset")
strSQL = "SELECT tblUsers.Password FROM tblUsers WHERE tblUsers.UserID ='" & Ucase(strUserName) & "'"
rsCheckUser.Open strSQL, strCon
If NOT rsCheckUser.EOF Then
If (Ucase(Request.Form("txtUserPass"))) = rsCheckUser("Password") Then
Set adoCon = Nothing
Set strCon = Nothing
Set rsCheckUser = Nothing
Response.Redirect"pag_user_aut.asp?name=&quot ; & Ucase(strUserName)
End If
End If

Set adoCon = Nothing
Set strCon = Nothing
Set rsCheckUser = Nothing
Response.Write("Ya Existe")

Saludos