This has been discussed over here a bit. In my case it helped to go to Appearance → Themes and deactivate and activate the theme again. Then this errormessage in the dashboard was gone and the website was loading again.
FWIW, this happened to me as well. Upgrading WP from r12817 to r13811 *and* upgrading the reCaptcha plugin too.
Well, by “upgrading” the latter I just renamed the old version to something else, not realizing that WP will still try to make a plugin from everything under wp-content/plugins – and now some functions were obiously doubled.
Strange error message though, neither the old nor the current reCaptcha version does anything with tinymce, dunno how this error came to be exactly.
HTH,
C.
