public class CustomDocking : RadDocking
{
    protected override void OnPropertyChanged(DependencyPropertyChangedEventArgs e)
    {
        base.OnPropertyChanged(e);
        if (e.Property.Name == nameof(PanesSource) && PanesSource is INotifyCollectionChanged observableCollection)
        {
            UnsubscribeCollectionChanged(observableCollection, this);
        }
    }

    public static void UnsubscribeCollectionChanged(INotifyCollectionChanged collection, object dockingInstance)
    {
        var handlerMethod = typeof(RadDocking).GetMethod("OnPanesSourceCollectionChanged", BindingFlags.NonPublic | BindingFlags.Instance);                        
        var handlerDelegate = Delegate.CreateDelegate(typeof(NotifyCollectionChangedEventHandler), dockingInstance, handlerMethod);

        var field = collection.GetType().GetField("CollectionChanged", BindingFlags.Instance | BindingFlags.NonPublic);
        var eventDelegate = (MulticastDelegate)field.GetValue(collection);

        var handlers = eventDelegate?.GetInvocationList().Where(d => d.Method == handlerMethod);
        bool shouldRemoveHandler = handlers.Count() > 1;                                
        if (shouldRemoveHandler)
        {
            typeof(INotifyCollectionChanged)
                .GetEvent("CollectionChanged")
                ?.RemoveMethod
                ?.Invoke(collection, new object[] { handlerDelegate });
        }
    }
}

var processing_provider = new Telerik.Windows.Documents.Flow.FormatProviders.Docx.DocxFormatProvider();

var document = processing_provider.Import(File.ReadAllBytes("C:\\Users\\test\\Downloads\\word1.docx"),null);
var bytes_ = processing_provider.Export(document, null);

var rtb_provider = new Telerik.Windows.Documents.FormatProviders.OpenXml.Docx.DocxFormatProvider();
var doc = rtb_provider.Import(bytes_);
radRichTextBox.Document = doc;