The XML Debugger plugin adds a number of filters, all with the same name. This leads to the following stack trace;
Actually, on reflection not a bug in the XMLDebugger (although all the filters have the same name ('rawDebugger'), each is applied to different filter chains.
This is caused by https://issues.igniterealtime.org/browse/OF-1582 (which meant that the PluginManagerListener was fired twice, so two attempts were made to add the filters.