it's INSERT ... ON DUPLICATE KEY UPDATE , not INSERT ON DUPLICATE KEY UPDATE SET

mysql-test/r/insert_update.result

 DROP TABLE IF EXISTS t1;
 CREATE TABLE t1 (a INT, b INT, c INT, UNIQUE (A), UNIQUE(B));
 INSERT t1 VALUES (1,2,10), (3,4,20);
-INSERT t1 VALUES (5,6,30) ON DUPLICATE KEY UPDATE SET c=c+100;
+INSERT t1 VALUES (5,6,30) ON DUPLICATE KEY UPDATE c=c+100;
 SELECT * FROM t1;
 a	b	c
 1	2	10
 3	4	20
 5	6	30
-INSERT t1 VALUES (5,7,40) ON DUPLICATE KEY UPDATE SET c=c+100;
+INSERT t1 VALUES (5,7,40) ON DUPLICATE KEY UPDATE c=c+100;
 SELECT * FROM t1;
 a	b	c
 1	2	10
 3	4	20
 5	6	130
-INSERT t1 VALUES (8,4,50) ON DUPLICATE KEY UPDATE SET c=c+1000;
+INSERT t1 VALUES (8,4,50) ON DUPLICATE KEY UPDATE c=c+1000;
 SELECT * FROM t1;
 a	b	c
 1	2	10
 3	4	1020
 5	6	130
-INSERT t1 VALUES (1,4,60) ON DUPLICATE KEY UPDATE SET c=c+10000;
+INSERT t1 VALUES (1,4,60) ON DUPLICATE KEY UPDATE c=c+10000;
 SELECT * FROM t1;
 a	b	c
 1	2	10010
 3	4	1020
 5	6	130
-INSERT t1 VALUES (1,9,70) ON DUPLICATE KEY UPDATE SET c=c+100000, b=4;
+INSERT t1 VALUES (1,9,70) ON DUPLICATE KEY UPDATE c=c+100000, b=4;
 Duplicate entry '4' for key 2
 SELECT * FROM t1;
 a	b	c
@@ -34,14 +34,14 @@ a	b	c
 5	6	130
 TRUNCATE TABLE t1;
 INSERT t1 VALUES (1,2,10), (3,4,20);
-INSERT t1 VALUES (5,6,30), (7,4,40), (8,9,60) ON DUPLICATE KEY UPDATE SET c=c+100;
+INSERT t1 VALUES (5,6,30), (7,4,40), (8,9,60) ON DUPLICATE KEY UPDATE c=c+100;
 SELECT * FROM t1;
 a	b	c
 1	2	10
 3	4	120
 5	6	30
 8	9	60
-INSERT t1 SET a=5 ON DUPLICATE KEY UPDATE SET b=0;
+INSERT t1 SET a=5 ON DUPLICATE KEY UPDATE b=0;
 SELECT * FROM t1;
 a	b	c
 1	2	10

mysql-test/t/insert_update.test

 DROP TABLE IF EXISTS t1;
 CREATE TABLE t1 (a INT, b INT, c INT, UNIQUE (A), UNIQUE(B));
 INSERT t1 VALUES (1,2,10), (3,4,20);
-INSERT t1 VALUES (5,6,30) ON DUPLICATE KEY UPDATE SET c=c+100;
+INSERT t1 VALUES (5,6,30) ON DUPLICATE KEY UPDATE c=c+100;
 SELECT * FROM t1;
-INSERT t1 VALUES (5,7,40) ON DUPLICATE KEY UPDATE SET c=c+100;
+INSERT t1 VALUES (5,7,40) ON DUPLICATE KEY UPDATE c=c+100;
 SELECT * FROM t1;
-INSERT t1 VALUES (8,4,50) ON DUPLICATE KEY UPDATE SET c=c+1000;
+INSERT t1 VALUES (8,4,50) ON DUPLICATE KEY UPDATE c=c+1000;
 SELECT * FROM t1;
-INSERT t1 VALUES (1,4,60) ON DUPLICATE KEY UPDATE SET c=c+10000;
+INSERT t1 VALUES (1,4,60) ON DUPLICATE KEY UPDATE c=c+10000;
 SELECT * FROM t1;
 -- error 1062
-INSERT t1 VALUES (1,9,70) ON DUPLICATE KEY UPDATE SET c=c+100000, b=4;
+INSERT t1 VALUES (1,9,70) ON DUPLICATE KEY UPDATE c=c+100000, b=4;
 SELECT * FROM t1;
 TRUNCATE TABLE t1;
 INSERT t1 VALUES (1,2,10), (3,4,20);
-INSERT t1 VALUES (5,6,30), (7,4,40), (8,9,60) ON DUPLICATE KEY UPDATE SET c=c+100;
+INSERT t1 VALUES (5,6,30), (7,4,40), (8,9,60) ON DUPLICATE KEY UPDATE c=c+100;
 SELECT * FROM t1;
-INSERT t1 SET a=5 ON DUPLICATE KEY UPDATE SET b=0;
+INSERT t1 SET a=5 ON DUPLICATE KEY UPDATE b=0;
 SELECT * FROM t1;
 DROP TABLE t1;

sql/sql_yacc.yy

@@ -3063,7 +3063,7 @@ opt_insert_update:
               YYABORT;
             }
           }
-          KEY_SYM UPDATE_SYM SET update_list
+          KEY_SYM UPDATE_SYM update_list
         ;
 
 /* Update rows in a table */