One short-term solution for mojarra would be to not register a new onKeyUp handler if a debug handler is already registered (also stored in a global variable). However, this would not address the Portlet case, where the debug element should probably add onKeyUp to just the parent container.
Assigning to Mircea for comment and suggestions (I'm not sure whether the mojarra would will be most interested in a short-term fix or a Portlet solution). It should be possible to reproduce this bug in mojarra by altering the test case to always update the element containing the ui:debug via Ajax.
can you please confirm whether it occurs without ICEfaces (just jsf2.0)?