var textBlock = new TextBlock
            {
                Text = " [" + item.Name + "] ",
                TextAlignment = TextAlignment.Center,
                Foreground = new SolidColorBrush(Color.FromArgb(0xff, 0xff, 0x00, 0x00)),
            };
 
            InlineUIContainer inlineUiContainer = new InlineUIContainer(textBlock, new Size(10, 10));
            
            rrtb.InsertInline(inlineUiContainer);
            rrtb.Focus();

Thank you for bringing this issue to our attention. The problem is that with text blocks, the text is kept in two places - in the Text property and in the Inlines collection (as a Run in your case). The serializer we use serializes both properties and on import the text gets duplicated.
Most probably we will revise the way XamlFormatProvider works and hopefully, we will manage to provide the means that will enable your scenario in the upcoming Q1.

We have not fixed this problem yet and I cannot provide you with a specific timeline of when it would be fixed, as it has not been scheduled for revision yet. We have quite a few features and issues of higher priority on the to-do list for Q2 and we have our hands pretty much full.
However, as documents exported to XAML are valid XML, you can open them using XDocument and clear the value of the Inlines property or the Text. Then on load in the rich text box the text will not be duplicated.
I hope this helps.