INSERT INTO .. ON DUPLICATE KEY UPDATE for multiple items

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

Too low on rep for comment, but I wanted to add a slightly more complex syntax that was inspired by @ʞɔıu response. To update multiple fields on duplicate key:

INSERT INTO t (t.a, t.b, t.c, t.d)
VALUES ('key1','key2','value','valueb'), ('key1','key3','value2','value2b')
ON DUPLICATE KEY UPDATE
t.c = VALUES(t.c),
t.d = VALUES(t.d)

Hope that helps someone out there looking to perform bulk insert with multiple on duplicate key update. The syntax escaped me.

After MySQL 8.0.19, you can use as keyword, for example:

INSERT INTO t1 (a,b,c) VALUES (1,2,3),(4,5,6) AS new
  ON DUPLICATE KEY UPDATE c = new.a+new.b;

or

INSERT INTO t1 (a,b,c) VALUES (1,2,3),(4,5,6) AS new(m,n,p)
  ON DUPLICATE KEY UPDATE c = m+n;

ref: https://dev.mysql.com/doc/refman/8.0/en/insert-on-duplicate.html