I believe I found the cause of the duplicate class names appearing in the Apply CSS Class drop down list of the RadEditor.

I was adding my CSS class definitions to both the /Providers/HtmlEditorProviders/Telerik/EditorOverride.css file and the /Portals/_default/Skins/Skin_Name/skin.css file.  The use of the /Providers/HtmlEditorProviders/Telerik/EditorOverride.css file appeared to be causing the issue.

Thanks to a tip from the video series at dnncreative.com, I learned that I was missing the following property in my /Providers/HtmlEditorProviders/Telerik/Config/ConfigDefault.xml file for the RadEditor.  

[PortalRoot]/Name_of_Editor_CSS_File.css

Once I did the following list of items, everything worked perfectly:

• Added the above property to the /Providers/HtmlEditorProviders/Telerik/Config/ConfigDefault.xml file
• Cleared out the CSS class definitions from the /Providers/HtmlEditorProviders/Telerik/EditorOverride.css
• Added the CSS class definitions to the [PortalRoot]Name_of_Editor_CSS_File.css file 

I hope this helps others avoid the same issues I was experiencing while using custom classes in the RadEditor.

I am not sure what I was doing wrong with the Telerik RadEditor, I followed the tutorials from dnncreative.com, but my Apply CSS Class drop down list remained empty.

I also tried the dnnwerk provider, but in that case I had the opposite result: the Apply CSS Class showed a very huge list, and I would like only a few to use there. It seems that dnnwerk provider collects all my defaults styles from everywhere (portal, skin, containers etc)