må max deltage en gan pr. tlf. nr!

Hvordan kan jeg lave en begrænsning så man kun kan skrive i databasen en gang pr. tlf. nr?
koden sersådan ud:

<%

dim write

set conn = server.createobject("ADODB.connection")
conn.Open "DRIVER={Microsoft Access Driver (*.mdb)}; DBQ= " &server.mappath("../fpdb/db.mdb")


set write = server.createobject("ADODB.recordset")
wSQL ="SELECT * FROM db"
write.CursorType = 2
write.LockType = 3
write.open wSQL, conn

write.AddNew

write.fields("Navn") = request.form("Navn")
write.fields("Adr") = request.form("Adr")
write.fields("Mail") = request.form("Mail")
write.fields("Tlf") = request.form("Tlf")
write.fields("By") = request.form("By")
write.fields("sv1") = request.form("sv1")
write.fields("sv2") = request.form("sv2")
write.fields("sv3") = request.form("sv3")

write.Update

write.close
set write = Nothing
conn.close
set conn = Nothing

Response.Redirect "tak.htm"
%>