Highlighting duplicate values - tsql

Question

I have the following script:
SELECT
Reference.ReferenceNumber AS [Reference.ReferenceNumber)
Reference.LastName
Reference.FirstName
Address.ReferenceNumber AS [Address.ReferenceNumber]
Address.Address1
Address.Address2
Address.Address3
Address.Address4
Address.ZipCode
Telephone.ReferenceNumber AS [Telephone.ReferenceNumber]
Telephone.TelephoneNumber
Email.ReferenceNumber AS [Email.ReferenceNumber]
Email.EmailAddress
FROM
Reference
INNER JOIN Address
ON Reference.ReferenceNumber = Reference.ContactNumber
LEFT OUTER JOIN Telephone
ON Reference.ReferenceNumber = Telephone.ReferenceNumber
LEFT OUTER Join Email
ON Reference.ReferenceNumber = Email.ReferenceNumber
This pulls through all customers, plus their addresses, email and phone numbers.
However some have duplicate surnames and addresses.
Please can you advise an amendment to the script that would show which records were duplicates? Ideally I'd like the output to show where the surname, address 1 and the zipcode were identical and restrict the output to the duplicate rows only.
Many thanks.

Answer 1

something like
SELECT
Reference.ReferenceNumber AS [Reference.ReferenceNumber),
case when vrln.c is null then Reference.LastName else 'dup: ' + Reference.LastName end as LastName,
Reference.FirstName,
Address.ReferenceNumber AS [Address.ReferenceNumber],
Address.Address1,
Address.Address2,
Address.Address3,
Address.Address4,
Address.ZipCode,
Telephone.ReferenceNumber AS [Telephone.ReferenceNumber],
Telephone.TelephoneNumber,
Email.ReferenceNumber AS [Email.ReferenceNumber],
Email.EmailAddress
FROM
Reference
INNER JOIN Address
ON Reference.ReferenceNumber = Reference.ContactNumber
LEFT OUTER JOIN Telephone
ON Reference.ReferenceNumber = Telephone.ReferenceNumber
LEFT OUTER Join Email
ON Reference.ReferenceNumber = Email.ReferenceNumber
left join (
select rln.LastName, add.Address1, add.ZipCode, count(rln.LastName) as c
from
Reference rln
INNER JOIN Address add
ON rln.ReferenceNumber = add.ReferenceNumber
group by rln.LastName, add.Address1, add.ZipCode
having count(rln.LastName) > 1
) vrln on Reference.LastName = vrln.LastName and Address.Address1 = vrln.Address1 and Address.ZipCode = vrln.ZipCode
where vrln.c is not null
but, to find duplicates :
select rln.LastName, add.Address1, add.ZipCode, count(rln.LastName) as c
from
Reference rln
INNER JOIN Address add
ON rln.ReferenceNumber = add.ReferenceNumber
group by rln.LastName, add.Address1, add.ZipCode
having count(rln.LastName) > 1
is enough