SQL Server 2014

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select @@version;


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Microsoft SQL Server 2014 (SP3) (KB4022619) - 12.0.6024.0 (X64) 
 Sep  7 2018 01:37:51 
 Copyright (c) Microsoft Corporation
 Express Edition (64-bit) on Windows NT 6.3 <X64> (Build 17763: ) (Hypervisor)



CREATE TABLE #TestEmailTable (
  companyId INT,
  username VARCHAR(255),
  emailAddress VARCHAR(255)
);
INSERT #TestEmailTable VALUES 
   (570599, 'emailAddress@bob.com', 'emailAddress@bob.com'),
   --(570599, 'brave@knight.net', 'emailAddress@bob.com'),
   (570589, 'hello@bob.com', 'goodbye@gmail.com'),
   (109350, 'dupedAddress@bob2.com', 'homeAddress@bob2.com'),
   (109350, 'anotherAddress@bob4.com', 'dupedAddress@bob2.com');
4 rows affected

with
  T as (
    select
      row_number() over(partition by companyId
                        order by (select null)) as rowid,
      *
    from #TestEmailTable
  )
select
  companyid, username, emailAddress
from T
where exists(
        select *
        from T as b
        where b.companyId = T.companyId and
              b.emailAddress = T.username and
              b.rowid != T.rowid
      );


companyid
username
emailAddress


109350
dupedAddress@bob2.com
homeAddress@bob2.com


select
  companyid, username, emailAddress
from #TestEmailTable as a
where exists(
        select *
        from #TestEmailTable as b
        where b.companyId = a.companyId and
              b.emailAddress = a.username and
              b.%%physloc%% != a.%%physloc%%
      );


companyid
username
emailAddress


109350
dupedAddress@bob2.com
homeAddress@bob2.com


select
  companyid, username, emailAddress
from #TestEmailTable as a
where exists(
        select *
        from #TestEmailTable as b
        where b.companyId = a.companyId and
              b.username != a.username and
              b.emailAddress = a.username
      );


companyid
username
emailAddress


109350
dupedAddress@bob2.com
homeAddress@bob2.com