User blog comment:Rappy 4187/Technical Update: March 10, 2015/@comment-452-20150312181303

Not mentioned in this blog post:
 * MediaWiki:Chat.js, MediaWiki:Chat.css, User:(you)/chat.js and User:(you)/chat.css are now loaded automatically.

This means that anyone who previously had MediaWiki:Chat.js and User:(you)/chat.js loaded through hacks now has them loaded twice, and all bindings are set twice.