To reproduce easily, use Firefox to view the "Grouping" ace:dataTable sample in showcase. With the JS error console open, slowly click the same grouped row to expand, then collapse, then expand, etc. the row until a JS error appears in the console.

For example, clicking the "Swansea" row (6th from the top) to expand worked fine for me, then clicking again to collapse failed with:

"[window] [Thu, 08 May 2014 15:53:26 GMT] Error [status: malformedXML code: 200]: During update: form:carTable_exp_5 not found
<?xml version='1.0' encoding='UTF-8'?>
<partial-response><changes><update id="form:carTable_row_5"><![CDATA[<tr class=" ui-datatable-odd  " id="form:carTable_row_5" tabindex="0"><td class="null ui-col-0"><div class="ui-row-toggler-parent"><a class="ui-icon ui-row-panel-toggler ui-icon-circle-triangle-e" tabindex="0"></a></div></td><td class="null ui-col-1"><span id="form:carTable:5:idCell">19</span></td><td class="null ui-col-2"><span id="form:carTable:5:nameCell">Swansea</span></td><td class="null ui-col-4"><span id="form:carTable:5:weightCell">1466</span></td><td class="null ui-col-6"><span id="form:carTable:5:mpgCell">11.72</span></td><td class="null ui-col-7"><span id="form:carTable:5:costCell">$19,266.56</span></td></tr>]]></update><update id="form:carTable_exp_5"><![CDATA[<tr id="form:carTable_exp_5" style="display:none;"></tr>]]></update><update id="javax.faces.ViewState"><![CDATA[4426365689436566047:5046903797931448401]]></update><extension aceCallbackParam="validationFailed">{"validationFailed":false}</extension></changes></partial-response>"

EE-3.3.0-maintenance branch rev. 40984:
This error occurs also when leaving Lazy Loading demo by clicking on Grouping link (FF28).

EE-3.3.0-maintenance branch rev. 40984/ Firefox28:
showcase -> DataTable:
> MultiRow Header/Grouping: server-side error, and "malformedXML code: 200" browser-console error when after sorting columns on MultiRow Header, click on Grouping:

May 09, 2014 11:31:17 AM com.sun.faces.context.AjaxExceptionHandlerImpl handlePartialResponseError
SEVERE: java.lang.IllegalStateException: Component ID form:carTable:j_idt143 has already been found in the view.
at com.sun.faces.util.Util.checkIdUniqueness(Util.java:879)
at com.sun.faces.util.Util.checkIdUniqueness(Util.java:863)
at com.sun.faces.util.Util.checkIdUniqueness(Util.java:863)
at com.sun.faces.util.Util.checkIdUniqueness(Util.java:863)
at com.sun.faces.util.Util.checkIdUniqueness(Util.java:863)
at com.sun.faces.util.Util.checkIdUniqueness(Util.java:863)
at com.sun.faces.util.Util.checkIdUniqueness(Util.java:863)
at com.sun.faces.util.Util.checkIdUniqueness(Util.java:863)
at com.sun.faces.util.Util.checkIdUniqueness(Util.java:863)
at com.sun.faces.application.view.FaceletPartialStateManagementStrategy.saveView(FaceletPartialStateManagementStrategy.java:461)
at com.sun.faces.application.StateManagerImpl.saveView(StateManagerImpl.java:89)
at javax.faces.application.StateManager.getViewState(StateManager.java:552)
at org.icefaces.impl.context.DOMPartialViewContext.renderState(DOMPartialViewContext.java:702)
..........

Re-tested with EE-3.3.0-maintenance branch / IF4 trunk rev. 40990 (Mojarra and MyFaces) and none of the errors could be reproduced anymore.

Resolving issue as fixed, after ICE-9542 commits that fixed this issue as well.