I have a course where a user has multiple roles.
I fetch the users with specific roles (including the roles which my user has) of a course:
$courseteachers = get_role_users(array('1', '2', '3'), context_course::instance(42), true);

Then there is a debug message complaining about the SQL query:

Did you remember to make the first column something unique in your call to get_records? Duplicate value '458' found in column 'id'.
line 780 of /lib/dml/pgsql_native_moodle_database.php: call to debugging()
line 3974 of /lib/accesslib.php: call to pgsql_native_moodle_database->get_records_sql()
line 395 of /blocks/course_overview_campus/block_course_overview_campus.php: call to get_role_users()
line 292 of /blocks/moodleblock.class.php: call to block_course_overview_campus->get_content()
line 238 of /blocks/moodleblock.class.php: call to block_base->formatted_contents()
line 951 of /lib/blocklib.php: call to block_base->get_content_for_output()
line 1003 of /lib/blocklib.php: call to block_manager->create_block_contents()
line 355 of /lib/outputrenderers.php: call to block_manager->ensure_content_created()
line 45 of /theme/clean/layout/columns3.php: call to core_renderer->standard_head_html()
line 860 of /lib/outputrenderers.php: call to include()
line 790 of /lib/outputrenderers.php: call to core_renderer->render_page_layout()
line 153 of /my/index.php: call to core_renderer->header()

I think get_role_users() should be optimized to get rid of this debug message.