Implementing a one to one (1:1) relationship between Person and User Scenario description
I have this person table as super parent (or supertype),
id
firstname
lastname
email
telephone
...
...

and user table as a child (or subtype)
id
person_id (FK)
password
username
screenname
...
... 

They must be in a one-to-one (1:1) relationship, because an user cannot be repeated twice, and so a particular email value in a given person row must not be repeated twice.
Then I have this message table which stores messages from anyone,
id
firstname
lastname
email
telephone
subject
content
...
...

but you can see that firstname, lastname, email, telephone are duplicated in the message table.
So, I am thinking to refer it to person table like this below,
id
person_id
subject
content
...

but then it does not seem right, as a person with the same email, name, etc. can send a message to me as many times as they want, so the details he/she provides can be repeated.
Questions

So, should I make message as a child of person the parent or they should be separate entities?

Or, are there any better suggestions to solve this problem?
I have a person table. 


Fine (just changed the name of the PK column):

person
------
person_id    PK
firstname
lastname
email
telephone
...
...



  and a user table as a child. They must be 1:1 relationship, because an user cannot be repeated twice.


If the relationship is 1:1 (assuming that person is the supertpe and user is the subtype, you can handle this with having the same column both as Primary Key and Foreign Key to person:

user
----
person_id  PK  FK to person(person_id)
password
username
screenname
...
... 



  and so the email in the person row must not be repeated twice.


Add a UNIQUE constraint in the person.email column.


  Then I have this message table which stores messages from anyone,
  but you can see that firstname,lastname, email,telephone are duplicated in message table.
  so I am thinking to refer it to person table like this below.


Fine (adjusted to previous changes). But you also need to store who sends a message and who is the receiver:

message
-------
message_id   PK
sender_id    FK  to user(person_id)         --- or to person(person_id)
receiver_id  FK  to user(person_id)         --- that depends on your requirements
subject
content
...