Beginning with MySQL 8.0.19 you can use an alias for that row (see reference).

INSERT INTO beautiful (name, age)
    VALUES
    ('Helen', 24),
    ('Katrina', 21),
    ('Samia', 22),
    ('Hui Ling', 25),
    ('Yumie', 29)
    AS new
ON DUPLICATE KEY UPDATE
    age = new.age
    ...

For earlier versions use the keyword VALUES (see reference, deprecated with MySQL 8.0.20).

INSERT INTO beautiful (name, age)
    VALUES
    ('Helen', 24),
    ('Katrina', 21),
    ('Samia', 22),
    ('Hui Ling', 25),
    ('Yumie', 29)
ON DUPLICATE KEY UPDATE
    age = VALUES(age),
     ...

Building a REPLACE INTO query is just as simple as building an INSERT ... ON DUPLICATE KEY UPDATE. Note that there is a difference between REPLACE INTO AND ON DUPLICATE KEY UPDATE.

Code:

$columns = ['id', 'date'];
$data = [
    [79, '2018-12-01'],
    [78, '2018-10-01'],
    [76, '2018-06-01'],
    [80, '2019-01-01']
];

$db = JFactory::getDbo();
try {
    foreach ($data as &$row) {
        $row = (int)db->q($row[1]); // flatten 2-dim array and apply security techniques
    }
    $query = $db->getQuery(true)
                ->insert($db->qn('#__tablename'))
                ->columns(columns))
                ->values($data);
    $db->setQuery(substr_replace($query, 'REPLACE', 0, 8));    // swap INSERT for REPLACE
    // in other words: $db->setQuery(str_replace("\r\nINSERT", "REPLACE", $query));
    // or:             $db->setQuery(preg_replace("~^\s+INSERT~", "REPLACE", $query));
    // Joomla puts \r\n at the start of the query; see via var_dump($db) after setQuery()
    /*
    echo "<pre>";
        print_r($db);   // see the updated sql object
    echo "</pre>";
    */
    $db->execute();
} catch (Exception $e) {
    echo  "Syntax Error: " , $e->getMessage();  // never show php's error message to the public
}
echo "<div>" , $db->getAffectedRows() , " row(s) inserted</div>";

This is the query built:

REPLACE INTO `#__tablename`
(`id`,`date`) VALUES 
(79, '2018-12-01'),(78, '2018-10-01'),(76, '2018-06-01'),(80, '2019-01-01')

This will ALWAYS print the same number of affected rows for your batch of four entries:

4 row(s) inserted

As for ON DUPLICATE KEY UPDATE..., this is as simple as appending the clause to the end of the Joomla-generated query and including VALUES() syntax. (only replace the $db->setQuery() line from the above snippet)

$db->setQuery($query . " ON DUPLICATE KEY UPDATE `date` = VALUES(`date`)");

to create this query:

INSERT INTO `#__tablename`
(`id`,`date`) VALUES 
(79, '2018-12-01'),(78, '2018-10-01'),(76, '2018-06-01'),(80, '2019-01-01')
ON DUPLICATE KEY UPDATE `date` = VALUES(`date`)

This type of query behaves differently and gives different feedback in terms of affected rows.

When identical rows exist in the database table for all of the rows that you intend to add, you will see 0 affected rows. If the PRIMARY ID already exists -- let's say 1 row has a different date value than what is in the query -- you will see 2 affected rows (I have run this on my own localhost and despite other things I've read, this must be removing, then adding). So this kind of information can be either very helpful or very confusing depending on what you are trying to determine. Imagine if you have all four ids represented in your table already and two of the four rows need the date value to be updated. After the query executes, you will be notified that 4 rows have been affected.

Here is a relevant answer that I posted a few years ago on StackOverflow.

you forgot the values() key word

INSERT INTO `buoy_stations` (`id`, `coords`, `name`, `owner`, `pgm`, `met`, `currents`)
VALUES 
('00922', 'Point(30,-90)','name 1','owner 1','pgm 1','y','y'),
('00923', 'Point(30,-90)','name 2','owner 2','pgm 2','y','y'),
('00924', 'Point(30,-90)','name 3','owner 3','pgm 3','y','y'),
('00925', 'Point(30,-90)','name 4','owner 4','pgm 4','y','y'),
('00926', 'Point(30,-90)','name 5','owner 5','pgm 5','y','y')
ON DUPLICATE KEY
        UPDATE coords=values(coords), name=values(name), owner=values(owner), pgm=values(pgm), met=values(met), currents=values(currents);

INSERT INTO tb (name, date, stat1, stat2, stat3)
VALUES (?, ?, ?, ?, ?)
ON DUPLICATE KEY UPDATE stat1 = stat1 + VALUES(stat1), stat2 = stat2 + VALUES(stat2), stat3 = stat3 + VALUES(stat3)

Use the VALUES() function

INSERT INTO t (t.a, t.b, t.c)
VALUES ('key1','key2','value'), ('key1','key3','value2')
ON DUPLICATE KEY UPDATE
t.c = VALUES(t.c)

see http://dev.mysql.com/doc/refman/5.0/en/insert-on-duplicate.html

Well, this is old. But of course you only need to provide a value once, there's no reason to add it a second time in the query (which comes in handy for multiple inserts, or prepared statements):

INSERT INTO table1
  (col1, col2, col3, col4)
VALUES
  ('val1', 'val2', 'val3', 'val4')
ON DUPLICATE KEY UPDATE
  col2=VALUES(col2),
  col3=VALUES(col3) [,...]

Which has as advantage it will still work for a multiple insert statement:

INSERT INTO table1
  (col1, col2, col3, col4)
VALUES
  ('val1', 'val2', 'val3', 'val4'),
  ('val5', 'val6', 'val7', 'val8'),
  ('val9', 'val10', 'val11', 'val12')
ON DUPLICATE KEY UPDATE
  col2=VALUES(col2),
  col3=VALUES(col3) [,...]

A modern PHP example:

$sql = "INSERT INTO table1 (col1, col2, col3, col4) VALUES (?,?,?,?)
        ON DUPLICATE KEY UPDATE
        col2=VALUES(col2),
        col3=VALUES(col3),
        col4=VALUES(col4)";
$db->prepare($sql)->execute([$val1, $val2, $val3, $val4]);

Consider

INSERT INTO table (a,b,c) VALUES (1,2,3)
    -> ON DUPLICATE KEY UPDATE c=c+1;

If a and b are UNIQUE fields, UPDATE occurs on a = 1 OR b = 2. Also when condition a = 1 OR b = 2 is met by two or more entries, update is done only once.

Ex here table table with Id and Name UNIQUE fields

Id     Name     Value 
1      P        2 
2      C        3 
3      D        29 
4      A        6

If query is

INSERT INTO table (Id, Name, Value)
VALUES (1, C, 7);

then we get

Id     Name     Value 
1      P        2 
2      C        3 
3      D        29 
4      A        6
1      C        7

which violates uniqueness of Id and Name. Now with

INSERT INTO table (Id, Name, Value)
VALUES (1, C, 7)
ON DUPLICATE KEY UPDATE Value = 7;

we get

Id     Name     Value 
1      P        7 
2      C        7 
3      D        29 
4      A        6

Behavior on multiple keys is the following

UPDATE in ON DUPLICATE KEY UPDATE is performed if one of the UNIQUE field equals the value to be inserted. Here, UPDATE is performed on Id = 1 OR Name = C. It is equivalent to

UPDATE table 
SET Value = 7
WHERE Id = 1 OR Name = C;

What if I want one update only, for either key

Can use UPDATE statement with LIMIT keyword

UPDATE table 
SET Value = 7
WHERE Id = 1 OR Name = C
LIMIT 1;

which will give

Id     Name     Value 
1      P        7 
2      C        3 
3      D        29 
4      A        6

What if I want one update only if values for both keys are matched

One solution is to ALTER TABLE and make the PRIMARY KEY (or uniqueness) work on both fields.

ALTER TABLE table 
DROP PRIMARY KEY,
ADD PRIMARY KEY (Id, Name);

Now, on

INSERT INTO table (Id, Name, Value)
VALUES (1, C, 7)
ON DUPLICATE KEY UPDATE Value = 7;

we get

Id     Name     Value 
1      P        2 
2      C        3 
3      D        29 
4      A        6
1      C        7

since no duplicate (on both keys) is found.