ON DUPLICATE KEY UPDATE value; inserting same values twice - php

Question

I am trying to insert some values and if there is a DUPLICATE KEY UPDATE table person. I am getting the insert of values but it is repeating the values instead of doing an update. The input fields are dynamic and values can be added or deleted. But I am trying to not have repeating values. How can insert new values and if there is a duplicate key to then update the fields?
Html/(jquery function not included generates more input fields)
<ul id="pq_entry_1" class="clonedSection">
<li>
<input id="person_fname_1" name="person_fname_1" placeholder="Person #1 - First Name" type="text" />
</li>
<li>
<input id="person_lname_1" name="person_lname_1" placeholder="Last Name" type="text" />
</li>
</ul>
<input type='button' id='btnAdd' value='add another Person' />
<input type='button' id='btnDel' value='delete Delete' />
PHP/Mysql Query
//Insert or Update Values
$f = 1;
while(isset($_POST['person_fname_' . $f]))
{
$person_fname = $_POST['person_fname_' . $f];
$person_lname = $_POST['person_lname_' . $f];
$query_init3 = "INSERT INTO person (academy_id, first_name, last_name) VALUES (:id,:person_fname,:person_lname)
ON DUPLICATE KEY UPDATE academy_id=:id, first_name=:person_fname, last_name=:person_lname";
$query_prep3 = $db_con->prepare($query_init3);
$query_prep3->execute(array(
"id" => $id,
"person_fname" => $person_fname,
"person_lname" => $person_lname
));
$f++;
}
Table after query:

Answer 1

All sort of duplication check SQL
REPLACE INTO
INSERT .. ON DUPLICATE
INSERT IGNORE
works when table has Primary key or UNIQUE index. I think there is no UNIQUE INDEX. (maybe person_id is PK for AUTO_INCREMENT)
so, could you post your person's create statement? If person has not UNIQUE(person_fname, person_lname) you should add it with following sql:
ALTER TABLE person ADD UNIQUE (person_fname, person_lname);