We are using 6.3.1 version of RAD Editor for SharePoint and using the complete toolbar provided by telerik.
Here the issue is about "Apply CSS Styles" dropdown, where the items are repeated atleast for 4 items. Attaching the document with the screenshot
We are following the approach given in the article http://www.telerik.com/help/aspnet-ajax/moss-sp2010-css_styles.html. Had our Custom Styles defined in one of the CSS file by overriding the classes in the below file.
/Program Files/Common Files/Microsoft Shared/web server extensions/14/Template/Layouts/1033/Styles/HtmlEditorStyles.css - Even in that case it should display twice - But it is displaying about 4 items.