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while exists (
  select 
    C.CONTRAGENT_NAME,
    PR.PHONE 
  from CONTRAGENTS C
    inner join CONTRAGENT_PROPERTIES PR on PR.CONTRAGENT_ID = C.CONTRAGENT_ID
  except select NAME,PHONE from IMPORT_SOURCE)
begin
  declare @IDENTITIES table (CONTRAGENT_ID int)
  insert CONTRAGENTS (
    CONTRAGENT_NAME,
    …)
  output inserted.CONTRAGENT_ID into @IDENTITIES
  select distinct
    NAME,
    …
  from IMPORT_SOURCE

  insert CONTRAGENT_PROPERTIES (
    …,
    PHONE,
    …)
  select
    …,
    S.PHONE
    …
  from @IDENTITIES I
    inner join CONTRAGENTS C on C.CONTRAGENT_ID = I.CONTRAGENT_ID
    cross apply (
      select top 1
      from IMPORT_SOURCE S
        left join (
          select
            C.CONTRAGENT_NAME,
            PR.PHONE 
          from CONTRAGENTS C
            inner join CONTRAGENT_PROPERTIES PR on PR.CONTRAGENT_ID = C.CONTRAGENT_ID
        ) TX on TX.CONTRAGENT_NAME = S.NAME and TX.PHONE = S.PHONE
      where TX.CONTRAGENT_NAME is null)
end