wp_mail has been declared by another process or plugin

Here is an update from Mandrill developer:

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Hello,

Problem is that NextGEN is loading the following file in its constructor:
include_once(path_join(ABSPATH, ‘wp-includes/pluggable.php’));

That files loads the whole library of WordPress pluggable functions which have not been loaded by the time it is included. Unfortunately wp_mail is one of those functions that gets loaded.

So plugins fail (wpMandrill or NextGEN or whatever) when they try to override any pluggable function after NextGEN has been declared/loaded. wpMandrill, at least, fails gratefully! 😉

They should try to makes NextGEN load after all other plugins have been loaded, or implement a lazy/delayed loading in their plugin.