Index: lams_tool_assessment/conf/language/lams/ApplicationResources_en_AU.properties
===================================================================
diff -u -r46e0b05aa6ff4798e51f86f140dbea83b22b798e -r731ac9bda6562e241a71eb71b18caeaa49aeefb5
--- lams_tool_assessment/conf/language/lams/ApplicationResources_en_AU.properties	(.../ApplicationResources_en_AU.properties)	(revision 46e0b05aa6ff4798e51f86f140dbea83b22b798e)
+++ lams_tool_assessment/conf/language/lams/ApplicationResources_en_AU.properties	(.../ApplicationResources_en_AU.properties)	(revision 731ac9bda6562e241a71eb71b18caeaa49aeefb5)
@@ -376,6 +376,6 @@
 label.correct  =Correct
 label.incorrect  =Incorrect
 label.answer.alternatives  =Answer alternatives
+label.someone.allocated.this.answer  =Sorry, someone has allocated this answer already
 
-
 #======= End labels: Exported 370 labels for en AU =====
Index: lams_tool_assessment/src/java/org/lamsfoundation/lams/tool/assessment/service/AssessmentServiceImpl.java
===================================================================
diff -u -r18b228bd194ec58143b063ea9e0329c5ed12c276 -r731ac9bda6562e241a71eb71b18caeaa49aeefb5
--- lams_tool_assessment/src/java/org/lamsfoundation/lams/tool/assessment/service/AssessmentServiceImpl.java	(.../AssessmentServiceImpl.java)	(revision 18b228bd194ec58143b063ea9e0329c5ed12c276)
+++ lams_tool_assessment/src/java/org/lamsfoundation/lams/tool/assessment/service/AssessmentServiceImpl.java	(.../AssessmentServiceImpl.java)	(revision 731ac9bda6562e241a71eb71b18caeaa49aeefb5)
@@ -27,6 +27,7 @@
 import java.lang.reflect.InvocationTargetException;
 import java.sql.Timestamp;
 import java.util.ArrayList;
+import java.util.Arrays;
 import java.util.Collection;
 import java.util.Date;
 import java.util.HashMap;
@@ -36,6 +37,7 @@
 import java.util.List;
 import java.util.Locale;
 import java.util.Map;
+import java.util.Optional;
 import java.util.Set;
 import java.util.SortedMap;
 import java.util.SortedSet;
@@ -1392,7 +1394,7 @@
     }
 
     @Override
-    public void allocateAnswerToOption(Long questionUid, Long targetOptionUid, Long previousOptionUid,
+    public Optional<Long> allocateAnswerToOption(Long questionUid, Long targetOptionUid, Long previousOptionUid,
 	    Long questionResultUid) {
 	AssessmentQuestion assessmentQuestion = assessmentQuestionDao.getByUid(questionUid);
 	QbQuestion qbQuestion = assessmentQuestion.getQbQuestion();
@@ -1402,6 +1404,15 @@
 
 	//adding
 	if (previousOptionUid.equals(-1L)) {
+	    //search for duplicates and, if found, return false 
+	    for (QbOption option : qbQuestion.getQbOptions()) {
+		String name = option.getName();
+		String[] alternatives = name.split("\r\n");
+		if (Arrays.asList(alternatives).contains(answer)) {
+		    return  Optional.of(option.getUid());
+		}
+	    }
+	    
 	    for (QbOption targetOption : qbQuestion.getQbOptions()) {
 		if (targetOption.getUid().equals(targetOptionUid)) {
 		    String name = targetOption.getName();
@@ -1521,6 +1532,8 @@
 	    //recalculate marks in all Scratchie activities, that use modified QbQuestion
 	    toolService.recalculateScratchieMarksForVsaQuestion(qbQuestion.getUid());
 	}
+	
+	return Optional.empty();
     }
 
     @Override
Index: lams_tool_assessment/src/java/org/lamsfoundation/lams/tool/assessment/service/IAssessmentService.java
===================================================================
diff -u -r670dd8e248dc4705c92031c79d05a99559e35024 -r731ac9bda6562e241a71eb71b18caeaa49aeefb5
--- lams_tool_assessment/src/java/org/lamsfoundation/lams/tool/assessment/service/IAssessmentService.java	(.../IAssessmentService.java)	(revision 670dd8e248dc4705c92031c79d05a99559e35024)
+++ lams_tool_assessment/src/java/org/lamsfoundation/lams/tool/assessment/service/IAssessmentService.java	(.../IAssessmentService.java)	(revision 731ac9bda6562e241a71eb71b18caeaa49aeefb5)
@@ -26,6 +26,7 @@
 import java.lang.reflect.InvocationTargetException;
 import java.util.List;
 import java.util.Map;
+import java.util.Optional;
 import java.util.Set;
 
 import org.lamsfoundation.lams.notebook.model.NotebookEntry;
@@ -424,7 +425,17 @@
      */
     QuestionSummary getQuestionSummary(Long contentId, Long questionUid);
 
-    void allocateAnswerToOption(Long questionUid, Long targetOptionUid, Long previousOptionUid, Long questionResultUid);
+    /**
+     * Allocate learner's answer into one of the available answer groups.
+     * 
+     * @param questionUid
+     * @param targetOptionUid
+     * @param previousOptionUid
+     * @param questionResultUid
+     * @return if present, it contains optionUid of the option group containing duplicate (added there presumably by
+     *         another teacher working in parallel)
+     */
+    Optional<Long> allocateAnswerToOption(Long questionUid, Long targetOptionUid, Long previousOptionUid, Long questionResultUid);
 
     /**
      * For export purposes
Index: lams_tool_assessment/src/java/org/lamsfoundation/lams/tool/assessment/web/controller/MonitoringController.java
===================================================================
diff -u -re6dc4db4137cfd6b07a4aa79711b9d12b39fb78e -r731ac9bda6562e241a71eb71b18caeaa49aeefb5
--- lams_tool_assessment/src/java/org/lamsfoundation/lams/tool/assessment/web/controller/MonitoringController.java	(.../MonitoringController.java)	(revision e6dc4db4137cfd6b07a4aa79711b9d12b39fb78e)
+++ lams_tool_assessment/src/java/org/lamsfoundation/lams/tool/assessment/web/controller/MonitoringController.java	(.../MonitoringController.java)	(revision 731ac9bda6562e241a71eb71b18caeaa49aeefb5)
@@ -28,6 +28,7 @@
 import java.util.Date;
 import java.util.LinkedList;
 import java.util.List;
+import java.util.Optional;
 import java.util.Set;
 import java.util.TimeZone;
 import java.util.TreeSet;
@@ -194,11 +195,18 @@
     }
     
     @RequestMapping("/allocateUserAnswer")
-    @ResponseStatus(HttpStatus.OK)
-    public void allocateUserAnswer(HttpServletRequest request, HttpServletResponse response,
+    @ResponseBody
+    public String allocateUserAnswer(HttpServletRequest request, HttpServletResponse response,
 	    @RequestParam Long questionUid, @RequestParam Long targetOptionUid, @RequestParam Long previousOptionUid,
 	    @RequestParam Long questionResultUid) {
-	service.allocateAnswerToOption(questionUid, targetOptionUid, previousOptionUid, questionResultUid);
+	Optional<Long> optionUid = service.allocateAnswerToOption(questionUid, targetOptionUid, previousOptionUid,
+		questionResultUid);
+
+	ObjectNode responseJSON = JsonNodeFactory.instance.objectNode();
+	responseJSON.put("isAnswerDuplicated", optionUid.isPresent());
+	responseJSON.put("optionUid", optionUid.orElse(-1L));
+	response.setContentType("application/json;charset=utf-8");
+	return responseJSON.toString();
     }
 
     @RequestMapping("/userSummary")
Index: lams_tool_assessment/web/pages/monitoring/parts/questionsummary.jsp
===================================================================
diff -u -r0a731bb87287def162bc6adc4be2ff872d2002dc -r731ac9bda6562e241a71eb71b18caeaa49aeefb5
--- lams_tool_assessment/web/pages/monitoring/parts/questionsummary.jsp	(.../questionsummary.jsp)	(revision 0a731bb87287def162bc6adc4be2ff872d2002dc)
+++ lams_tool_assessment/web/pages/monitoring/parts/questionsummary.jsp	(.../questionsummary.jsp)	(revision 731ac9bda6562e241a71eb71b18caeaa49aeefb5)
@@ -18,6 +18,9 @@
     			min-height: 110px;
     			padding: 10px;
 			}
+			.filtered {
+			    background-color: lightgrey;
+			}
 			.list-group-item {
 				cursor: pointer;
 			}
@@ -146,6 +149,7 @@
 	  			    new Sortable($(this)[0], {
 	  			    	group: 'shared',
 	  				    animation: 150,
+	  				  	filter: '.filtered', // 'filtered' class is not draggable
 	  				    //ghostClass: 'sortable-placeholder',
 	  				    //direction: 'vertical',
 	  					onStart: function (evt) {
@@ -162,18 +166,17 @@
 	  				            	previousOptionUid: $(evt.from).data("option-uid"),
 	  				            	questionResultUid: $(evt.item).data("question-result-uid")
 	  						    },
-	  				            type: 'post'
+	  				            method: 'post',
+	  				          	dataType: "json",
+		  				        success: function (data) {
+		  				            if (data.isAnswerDuplicated) {
+			  				        	alert("<fmt:message key="label.someone.allocated.this.answer" />");
+			  				        	$(evt.item).appendTo("#answer-group" + data.optionUid);
+			  				        	$(evt.item).addClass("filtered");
+				  				    }
+		  				        }
+	  				            	
 	  				       	});
-	  					},
-	  					store: {
-	  						set: function (sortable) {
-	  							//update all displayOrders in order to later save it as options' order
-	  							var order = sortable.toArray();
-	  							for (var i = 0; i < order.length; i++) {
-	  							    var optionIndex = order[i];
-	  							    $('input[name="optionDisplayOrder' + optionIndex + '"]').val(i+1);
-	  							}
-	  						}
 	  					}
 	  				});
 	  			});
@@ -278,14 +281,14 @@
 						<fmt:message key="label.answer.alternatives" />: 
 						${fn:replace(option0.name, newLineChar, ', ')}
 						
-						<div class="list-group col sortable-on" data-option-uid="${option0.uid}"></div>	
+						<div class="list-group col sortable-on" data-option-uid="${option0.uid}" id="answer-group${option0.uid}"></div>	
 					</div>
 					
 					<div class="col-sm-4 text-center">
 		            	<h4><fmt:message key="label.answer.queue" /></h4>
 	            		(<fmt:message key="label.drag.and.drop" />)	
 	            		
-	            		<div class="list-group col sortable-on" data-option-uid="-1">
+	            		<div class="list-group col sortable-on" data-option-uid="-1" id="answer-group-1">
 		            		<c:forEach var="questionResult" items="${questionSummary.notAllocatedQuestionResults}">
 		            			<div class="list-group-item" data-question-result-uid="${questionResult.uid}">
 		            				<lams:Portrait userId="${questionResult.assessmentResult.user.userId}"/>&nbsp;
@@ -314,7 +317,7 @@
 						<fmt:message key="label.answer.alternatives" />: 
 						${fn:replace(option1.name, newLineChar, ', ')}
 						
-						<div class="list-group col sortable-on" data-option-uid="${option1.uid}"></div>	
+						<div class="list-group col sortable-on" data-option-uid="${option1.uid}" id="answer-group${option1.uid}"></div>	
 					</div>
 				</div>
 				
@@ -332,7 +335,7 @@
 						<fmt:message key="label.answer.alternatives" />: 
 						${fn:replace(optionDto.name, newLineChar, ', ')}
 						
-						<div class="list-group col sortable-on" data-option-uid="${optionDto.uid}"></div>	
+						<div class="list-group col sortable-on" data-option-uid="${optionDto.uid}" id="answer-group${optionDto.uid}"></div>	
 					</div>
 					
 					<c:if test="${status.count % 3 == 0 || status.last}">