Index: wp-includes/class-wp-editor.php
===================================================================
--- wp-includes/class-wp-editor.php	(revision 18555)
+++ wp-includes/class-wp-editor.php	(working copy)
@@ -291,6 +291,7 @@
 
 					// load editor_style.css if the current theme supports it
 					if ( ! empty( $editor_styles ) && is_array( $editor_styles ) ) {
+						$editor_styles = array_unique( $editor_styles );
 						$mce_css = array();
 						$style_uri = get_stylesheet_directory_uri();
 						if ( ! is_child_theme() ) {