Description:
When running insert .. values(), () ... the warning count is reported two times - being 1 and being 0. Correct warning count is 1.

How to repeat:
drop table if exists a;

create table a (
        a int auto_increment primary key,
        b int not null
);

select * from a;

insert into a (a) values(0);

insert into a (a) values(0), (0); /* warning count in the 2nd line is 0! */

Indeed, number of warnings is a bit misleading:

mysql> insert into a (a) values(0);
Query OK, 1 row affected, 1 warning (0.00 sec)

mysql> insert into a (a) values(0), (0);
Query OK, 2 rows affected, 1 warning (0.00 sec)
Records: 2  Duplicates: 0  Warnings: 0

mysql> show warnings\G
*************************** 1. row ***************************
  Level: Warning
   Code: 1364
Message: Field 'b' doesn't have a default value
1 row in set (0.08 sec)

Posted by developer:
 
Fixed in 5.7.