And ofcourse you can create a CHECKSUM computed column which is calculated from all other fields. So you can determine all the duplicate records easily, they are all give the same checksum value.

But unfortunately CHECKSUM is 32 bit integers, so if you have too much records in your table it may give the same checksum value.

You can also use the

--create a checksum field to see duplicates

ALTER TABLE Phonebook

ADD chkValue AS checksum(first_name, last_name, phonenumber)

--select and/or delete duplicates

...

--create unique index so no more dup. can be inserted

CREATE INDEX chkUnique ON Phonebook (chkValue) WITH IGNORE_DUP_KEY

Don't forget, if you have thousands of records then there can be different rows that give the same checksum value.