calling add_editor_style() twice results in the css being added multiple times

[dd32]

In the event a Theme, and a Child theme, both call add_editor_style() the TinyMCE css list will double up:

content_css:"
http://localhost/branch/3.2/wp-content/themes/twentyeleven/editor-style.css,
http://localhost/branch/3.2/wp-content/themes/twentyeleven-child/editor-style.css,
http://localhost/branch/3.2/wp-content/themes/twentyeleven/editor-style.css,
http://localhost/branch/3.2/wp-content/themes/twentyeleven-child/editor-style.css"

(New lines added for readability)

Affects both trunk and 3.2, This does point out it's a "doing it wrong" to call the add_editor_style() in the child theme, but we should also array_unique() the $editor_styles array before searching for the files to enqueue IMO.

[dd32]

In [18571]:

Only include extra TinyMCE stylesheets once. props SergeyBiryukov. Fixes #18456