I cannot get this to duplicate ... can you provide me with a test form where this is the case?
Sure:
http://www.sendspace.com/file/vivck3
Or you can do next steps:
1. Create empty Windows Forms application
2. Add two RichTextBoxes to form
3. Add one ContextMenuStrip. Link two RichTextBoxes with this one ContextMenuStrip.
4. Write OnLoad: this.EnableControlExtensions();
5. Run. Try to open context menu in any of RichTextBoxes.

First time extTextBoxContextMenu_MenuOpening work ok. But second time (don't understand why it called two times) will be error because extTextBoxContextMenu.MenuSpellClickReturn == null.