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Indeed, mail merge in data-bound scenarios is a bit tricky. This comes from the fact that the binding updates the document so if you specify your item source in the constructor of the page, it will be cleared once the document is RadRichTextBox is changed.
In order to make sure the item source is set, you can subscribe to the SetupDocument event of the respective data provider:
sender, Telerik.Windows.Documents.FormatProviders.SetupDocumentEventArgs e)
In general, you might want to subscribe to the DocumentChanged event of RadRichTextBox and set the item source there as well, as this will ensure that when a new document is created with the NewDocumentCommand or loaded with the OpenDocumentCommand, the respective source will be set.
I hope this helps. Let us know if you have other questions or comments.