Author: Michael Aherne
MDL-49280 user: Config for allowing duplicate emails
http://git.moodle.org/gw?p=moodle.git;a=commit;h=bc9837241f0d04200fad70bcbaa1471bb9b03ca7
http://github.com/moodle/moodle/commit/bc9837241f0d04200fad70bcbaa1471bb9b03ca7
+ 3.0 en [allowuserstoshareemailaddresses,core_auth]
+ 3.0 en [allowuserstoshareemailaddresses_desc,core_auth]