Index: lams_tool_preview/src/java/org/lamsfoundation/lams/tool/peerreview/dao/PeerreviewUserDAO.java
===================================================================
diff -u -r9f2c1f957570c3885c34d8d162a1f14d82e3aba2 -rf99ba3decf8c8e2c60346de01726623939afcd99
--- lams_tool_preview/src/java/org/lamsfoundation/lams/tool/peerreview/dao/PeerreviewUserDAO.java	(.../PeerreviewUserDAO.java)	(revision 9f2c1f957570c3885c34d8d162a1f14d82e3aba2)
+++ lams_tool_preview/src/java/org/lamsfoundation/lams/tool/peerreview/dao/PeerreviewUserDAO.java	(.../PeerreviewUserDAO.java)	(revision f99ba3decf8c8e2c60346de01726623939afcd99)
@@ -35,6 +35,8 @@
 
     List<PeerreviewUser> getBySessionID(Long sessionId);
 
+    List<Long> getUserIdsBySessionID(Long sessionId);
+    
     List<PeerreviewUser> getByContentId(Long toolContentId);
 
     int getCountUsersBySession(final Long toolSessionId, final Long excludeUserId);
Index: lams_tool_preview/src/java/org/lamsfoundation/lams/tool/peerreview/dao/hibernate/PeerreviewUserDAOHibernate.java
===================================================================
diff -u -r9f2c1f957570c3885c34d8d162a1f14d82e3aba2 -rf99ba3decf8c8e2c60346de01726623939afcd99
--- lams_tool_preview/src/java/org/lamsfoundation/lams/tool/peerreview/dao/hibernate/PeerreviewUserDAOHibernate.java	(.../PeerreviewUserDAOHibernate.java)	(revision 9f2c1f957570c3885c34d8d162a1f14d82e3aba2)
+++ lams_tool_preview/src/java/org/lamsfoundation/lams/tool/peerreview/dao/hibernate/PeerreviewUserDAOHibernate.java	(.../PeerreviewUserDAOHibernate.java)	(revision f99ba3decf8c8e2c60346de01726623939afcd99)
@@ -37,6 +37,8 @@
 	    + " AS u WHERE u.userId =? AND u.session.sessionId=?";
     private static final String FIND_BY_SESSION_ID = "FROM " + PeerreviewUser.class.getName()
 	    + " AS u WHERE u.session.sessionId=?";
+    private static final String GET_USERIDS_BY_SESSION_ID = "SELECT userId FROM " + PeerreviewUser.class.getName()
+	    + " AS u WHERE u.session.sessionId=?";
     
     private static final String FIND_BY_CONTENT_ID = "FROM " + PeerreviewUser.class.getName()
 	    + " AS u WHERE u.session.peerreview.contentId=?";
@@ -69,6 +71,11 @@
     }
     
     @Override
+    public List<Long> getUserIdsBySessionID(Long sessionId) {
+	return this.getHibernateTemplate().find(GET_USERIDS_BY_SESSION_ID, sessionId);
+    }
+
+    @Override
     public List<PeerreviewUser> getByContentId(Long toolContentId) {
 	return this.getHibernateTemplate().find(FIND_BY_CONTENT_ID, toolContentId);
     }
Index: lams_tool_preview/src/java/org/lamsfoundation/lams/tool/peerreview/dbupdates/patch20150825.sql
===================================================================
diff -u
--- lams_tool_preview/src/java/org/lamsfoundation/lams/tool/peerreview/dbupdates/patch20150825.sql	(revision 0)
+++ lams_tool_preview/src/java/org/lamsfoundation/lams/tool/peerreview/dbupdates/patch20150825.sql	(revision f99ba3decf8c8e2c60346de01726623939afcd99)
@@ -0,0 +1,13 @@
+SET AUTOCOMMIT = 0;
+SET FOREIGN_KEY_CHECKS = 0;
+
+-- --------------------Put all sql statements below here-------------------------
+
+ALTER TABLE `tl_laprev11_user`
+ADD UNIQUE INDEX `prev11uniqusersession` (`user_id` ASC, `session_uid` ASC);
+
+-- --------------------Put all sql statements above here-------------------------
+-- If there were no errors, commit and restore autocommit to on
+COMMIT;
+SET AUTOCOMMIT = 1;
+SET FOREIGN_KEY_CHECKS = 1;
\ No newline at end of file
Index: lams_tool_preview/src/java/org/lamsfoundation/lams/tool/peerreview/service/IPeerreviewService.java
===================================================================
diff -u -r0e69531f65529363399c5472a6c42bae1aa88408 -rf99ba3decf8c8e2c60346de01726623939afcd99
--- lams_tool_preview/src/java/org/lamsfoundation/lams/tool/peerreview/service/IPeerreviewService.java	(.../IPeerreviewService.java)	(revision 0e69531f65529363399c5472a6c42bae1aa88408)
+++ lams_tool_preview/src/java/org/lamsfoundation/lams/tool/peerreview/service/IPeerreviewService.java	(.../IPeerreviewService.java)	(revision f99ba3decf8c8e2c60346de01726623939afcd99)
@@ -193,10 +193,15 @@
     PeerreviewUser getUser(Long uid);
     
     /**
+     * Trigger the user creation, based on the lesson/grouping class, if needed. This
+     * should be called by the web layer to trigger an update. Returns almost straight away -
+     * any updates are done in a separate thread.
+     * Returns true if a check/update is triggered, returns false if a check is already underway.
      * @param toolSessionId
      * @return
+     * @throws Throwable 
      */
-    List<PeerreviewUser> fetchUsersFromLesson(Long toolSessionId);
+    boolean fetchUsersFromLesson(Long toolSessionId) throws Throwable;
 
     /**
      * Returns whether activity is grouped and therefore it is expected more than one tool session.
Index: lams_tool_preview/src/java/org/lamsfoundation/lams/tool/peerreview/service/PeerreviewServiceImpl.java
===================================================================
diff -u -r3ccf2da662a461f879bd902e9bf0eb648acf9983 -rf99ba3decf8c8e2c60346de01726623939afcd99
--- lams_tool_preview/src/java/org/lamsfoundation/lams/tool/peerreview/service/PeerreviewServiceImpl.java	(.../PeerreviewServiceImpl.java)	(revision 3ccf2da662a461f879bd902e9bf0eb648acf9983)
+++ lams_tool_preview/src/java/org/lamsfoundation/lams/tool/peerreview/service/PeerreviewServiceImpl.java	(.../PeerreviewServiceImpl.java)	(revision f99ba3decf8c8e2c60346de01726623939afcd99)
@@ -25,6 +25,7 @@
 
 import java.util.ArrayList;
 import java.util.Collection;
+import java.util.Collections;
 import java.util.Date;
 import java.util.Hashtable;
 import java.util.Iterator;
@@ -33,7 +34,9 @@
 import java.util.Map;
 import java.util.Set;
 import java.util.SortedMap;
+import java.util.SortedSet;
 import java.util.TreeMap;
+import java.util.TreeSet;
 
 import javax.servlet.http.HttpServletRequest;
 
@@ -91,7 +94,7 @@
     private PeerreviewUserDAO peerreviewUserDao;
 
     private PeerreviewSessionDAO peerreviewSessionDao;
-
+    
     // tool service
     private PeerreviewToolContentHandler peerreviewToolContentHandler;
 
@@ -111,6 +114,12 @@
     
     private IRatingService ratingService;
 
+    private SortedSet<Long> creatingUsersForSessionIds;
+
+    PeerreviewServiceImpl() {
+	creatingUsersForSessionIds = Collections.synchronizedSortedSet(new TreeSet<Long>());
+    }
+    
     // *******************************************************************************
     // Service method
     // *******************************************************************************
@@ -302,38 +311,70 @@
     }
     
     @Override
-    public List<PeerreviewUser> fetchUsersFromLesson(Long toolSessionId) {
+    public boolean fetchUsersFromLesson(Long toolSessionId) throws Throwable {
 	//can we change lesson's class?
 	//boolean isGroupedActivity = isGroupedActivity(toolContentId);
 
-	PeerreviewSession session = getPeerreviewSessionBySessionId(toolSessionId);
-	Set<User> lessonUsers = toolService.getUsersFromGroupingActivity(toolSessionId);
-	List<PeerreviewUser> sessionUsers = peerreviewUserDao.getBySessionID(toolSessionId);
-	
-	//create all new users
-	for (User lessonUser: lessonUsers) {
-	    
-	    boolean isCorrespondingUserExist = false;
-	    for (PeerreviewUser sessionUser: sessionUsers) {
-		if (lessonUser.getUserId().longValue() == sessionUser.getUserId().longValue()) {
-		    isCorrespondingUserExist = true;
-		    break;
-		}
+	try {
+	    boolean wasNotInSetAlready = creatingUsersForSessionIds.add(toolSessionId);
+	    if ( ! wasNotInSetAlready ) {
+		log.debug("Peer Review: Already processing session "+toolSessionId);
+		return false;
 	    }
 
-	    if (!isCorrespondingUserExist) {
-		PeerreviewUser newUser = new PeerreviewUser(lessonUser.getUserDTO(), session);
-		createUser(newUser);
-	    }
-
+	    log.debug("Peer Review: Processing session "+toolSessionId);
+	    Thread t = new Thread(new UserCreateThread(toolSessionId));
+	    t.start();
+	    return true;
+	} catch (Throwable e) {
+	    creatingUsersForSessionIds.remove(toolSessionId);
+	    String message = e.getMessage() != null ? e.getMessage() : e.getClass().getName();
+	    this.log.error("Exception thrown creating Peer Review users for session "+toolSessionId+": "+message,e);
+	    e.printStackTrace();
+	    throw(e);
 	}
-	
-	return sessionUsers;
     }
-
+	    
     // *****************************************************************************
     // private methods
     // *****************************************************************************
+   private class UserCreateThread implements Runnable {
+	private Long toolSessionId;
+
+	private Logger log = Logger.getLogger(UserCreateThread.class);
+
+	public UserCreateThread(Long toolSessionId) {
+	    this.toolSessionId = toolSessionId;
+	}
+
+	public void run() {
+	    long start = System.currentTimeMillis();
+	    try {
+		PeerreviewSession session = getPeerreviewSessionBySessionId(toolSessionId);
+		Set<User> lessonUsers = toolService.getUsersFromGroupingActivity(toolSessionId);
+		List<Long> sessionUserIds = peerreviewUserDao.getUserIdsBySessionID(toolSessionId);
+		boolean needsUpdate = sessionUserIds.size() <= lessonUsers.size();
+
+		if ( needsUpdate ) {
+		    //create all new users
+		    for (User lessonUser: lessonUsers) {
+			if (!sessionUserIds.contains(lessonUser.getUserId().longValue())) {
+			    PeerreviewUser newUser = new PeerreviewUser(lessonUser.getUserDTO(), session);
+			    createUser(newUser);
+			}
+		    }
+		}
+		this.log.debug("UserCreateThread "+toolSessionId+": Update Took "+ (System.currentTimeMillis()-start) +"ms.");
+	    } catch (Throwable e) {
+		String message = e.getMessage() != null ? e.getMessage() : e.getClass().getName();
+		this.log.error("Exception thrown creating Peer Review users for session "+toolSessionId+": "+message,e);
+		e.printStackTrace();
+	    } finally {
+		creatingUsersForSessionIds.remove(toolSessionId);
+	    }
+	}
+   } // end Thread class
+		
     private Peerreview getDefaultPeerreview() throws PeerreviewApplicationException {
 	Long defaultPeerreviewId = getToolDefaultContentIdBySignature(PeerreviewConstants.TOOL_SIGNATURE);
 	Peerreview defaultPeerreview = getPeerreviewByContentId(defaultPeerreviewId);
Index: lams_tool_preview/src/java/org/lamsfoundation/lams/tool/peerreview/web/action/LearningAction.java
===================================================================
diff -u -r0e69531f65529363399c5472a6c42bae1aa88408 -rf99ba3decf8c8e2c60346de01726623939afcd99
--- lams_tool_preview/src/java/org/lamsfoundation/lams/tool/peerreview/web/action/LearningAction.java	(.../LearningAction.java)	(revision 0e69531f65529363399c5472a6c42bae1aa88408)
+++ lams_tool_preview/src/java/org/lamsfoundation/lams/tool/peerreview/web/action/LearningAction.java	(.../LearningAction.java)	(revision f99ba3decf8c8e2c60346de01726623939afcd99)
@@ -113,10 +113,11 @@
      * method run successfully.
      * 
      * This method will avoid read database again and lost un-saved resouce item lost when user "refresh page",
+     * @throws IOException 
      * 
      */
     private ActionForward start(ActionMapping mapping, ActionForm form, HttpServletRequest request,
-	    HttpServletResponse response) {
+	    HttpServletResponse response) throws IOException  {
 
 	IPeerreviewService service = getPeerreviewService();
 
@@ -174,9 +175,13 @@
 	    return mapping.findForward("defineLater");
 	}
 
-	// handle rating criterias
-	service.fetchUsersFromLesson(sessionId);
+	try {
+	    service.fetchUsersFromLesson(sessionId);
+	} catch ( Throwable e ) {
+	    throw new IOException(e);
+	}
 	
+	// handle rating criterias
 	boolean isCommentsEnabled = service.isCommentsEnabled(peerreview.getContentId());
 	sessionMap.put("isCommentsEnabled", isCommentsEnabled);
 
Index: lams_tool_preview/web/pages/learning/learning.jsp
===================================================================
diff -u -r0e69531f65529363399c5472a6c42bae1aa88408 -rf99ba3decf8c8e2c60346de01726623939afcd99
--- lams_tool_preview/web/pages/learning/learning.jsp	(.../learning.jsp)	(revision 0e69531f65529363399c5472a6c42bae1aa88408)
+++ lams_tool_preview/web/pages/learning/learning.jsp	(.../learning.jsp)	(revision f99ba3decf8c8e2c60346de01726623939afcd99)
@@ -203,6 +203,11 @@
 		function continueReflect(){
 			document.location.href='<c:url value="/learning/newReflection.do?sessionMapID=${sessionMapID}"/>';
 		}
+		
+		function refresh(){
+			document.location.href='<c:url value="/learning/start.do?mode=learner&toolSessionID=${toolSessionId}"/>';
+		}
+
     </script>
 </lams:head>
 <body class="stripes">
@@ -349,6 +354,11 @@
 		</c:if>
 
 		<c:if test="${mode != 'teacher'}">
+			<div class="space-bottom-top align-left">
+				<html:button property="FinishButton" styleId="finishButton" onclick="refresh()" styleClass="button">
+					<fmt:message key="label.refresh" />
+				</html:button>
+			</div>
 			<div class="space-bottom-top align-right" id="learner-submit">
 				<c:choose>
 					<c:when test="${peerreview.showRatingsLeftForUser}">