The issue is reproduced with Marathi IME. Type some letters, press and release shift, then press space. The result is that the current IME text is duplicated.
With our latest Q3 2014 release we introduced an API that allows implementing custom support for different IMEs. For further reference check the realted SDK demo: https://github.com/telerik/xaml-sdk/tree/master/RichTextBox/CustomCaret Regards, The Telerik Team