Windows LDAP server (a.k.a. Active Directory) does not differenciate alphabet of multi-byte character from alphabet of single-byte character. At least, Japanese Windows LDAP server does not differenciate them.
Under this environment, an user who inputs user name in the alphabet of multi-byte characters is successfully authenticated by LDAP server even though his or her user name actually consits of alphabet of single-byte characters in LDAP. In this situation, LDAP authentication plugin of moodle creates an user record for user name in the alphabet of multi-byte characters into moodle database when an user inputs user name in alphabet of multi-byte characters.
That is, an user record in moodle database might be created duplicatedly.
I attached my patch that fixes the problem, it might look like dirty though.