I have narrowed the conflict down to Mandrill and NextGEN Gallery by Photocrati. A quick search shows me that this sort of conflict has also arisen with other plugins and wp_mail. The error persists in 3.6 and 3.5.2
Any advice on how to fix this?
Mandrill: wp_mail has been declared by another process or plugin, so you won't be able to use Mandrill until the problem is solved.