When RTF documents are imported simultaneously in different threads, sometimes ArgumentException with message "An item with the same key has already been added" is thrown with the following call stack:

at System.Collections.Generic.Dictionary`2.Insert(TKey key, TValue value, Boolean add)
   at System.Collections.Generic.Dictionary`2.Add(TKey key, TValue value)
   at Telerik.Windows.Documents.FormatProviders.ReplaceStyleNameHandler.Add(String displayName, String name)
   at Telerik.Windows.Documents.FormatProviders.ReplaceStyleNameHandler.InitializeStyleDisplayNameToStyleNameDictionary()
   at Telerik.Windows.Documents.FormatProviders.ReplaceStyleNameHandler.get_StyleDisplayNameToStyleName()
   at Telerik.Windows.Documents.FormatProviders.ReplaceStyleNameHandler.TryGetStyleNamesByStyleName(String displayName, RadDocumentDefaultStyleInfo& defaultStyleInfo)
   at Telerik.Windows.Documents.FormatProviders.ReplaceStyleNameHandler.Replace(ReplaceStyleNameContext context)
   at Telerik.Windows.Documents.FormatProviders.Rtf.Import.RtfStyleImporter.DoVisitText(RtfText text)
   at Telerik.Windows.Documents.FormatProviders.Rtf.Import.RtfElementIteratorBase.VisitElement(RtfElement element, Boolean recursive)
   at Telerik.Windows.Documents.FormatProviders.Rtf.Import.RtfElementIteratorBase.VisitGroupChildren(RtfGroup group, Boolean recursive)
   at Telerik.Windows.Documents.FormatProviders.Rtf.Import.RtfStyleImporter.ImportStyleGroup(RtfGroup group)
   at Telerik.Windows.Documents.FormatProviders.Rtf.Import.RtfStylesTable.FillStyle(RtfGroup group, StyleType type, Boolean isDefault)
   at Telerik.Windows.Documents.FormatProviders.Rtf.Import.RtfStylesTable.DoVisitGroup(RtfGroup group)
   at Telerik.Windows.Documents.FormatProviders.Rtf.Import.RtfElementIteratorBase.VisitElement(RtfElement element, Boolean recursive)
   at Telerik.Windows.Documents.FormatProviders.Rtf.Import.RtfElementIteratorBase.VisitGroupChildren(RtfGroup group, Boolean recursive)
   at Telerik.Windows.Documents.FormatProviders.Rtf.Import.RtfStylesTable.ReadTable(RtfGroup group, RtfImportContext context)
   at Telerik.Windows.Documents.FormatProviders.Rtf.Import.RtfHandlers.RtfGroupHandlers.StylesTableHandler(RtfGroup group, RtfImportContext context)
   at Telerik.Windows.Documents.FormatProviders.Rtf.Import.RtfDocumentImporter.DoVisitGroup(RtfGroup group)
   at Telerik.Windows.Documents.FormatProviders.Rtf.Import.RtfElementIteratorBase.VisitElement(RtfElement element, Boolean recursive)
   at Telerik.Windows.Documents.FormatProviders.Rtf.Import.RtfElementIteratorBase.VisitGroupChildren(RtfGroup group, Boolean recursive)
   at Telerik.Windows.Documents.FormatProviders.Rtf.Import.RtfDocumentImporter.ImportRoot(RtfGroup rtfGroup)
   at Telerik.Windows.Documents.FormatProviders.Rtf.Import.RtfDocumentImporter.Import(Stream input, RtfImportSettings settings)
   at Telerik.Windows.Documents.FormatProviders.Rtf.RtfFormatProvider.Import(Stream input)
   at Telerik.Windows.Documents.FormatProviders.Rtf.RtfFormatProvider.Import(String input)

Fix available in LIB Version 2018.3.924.