Index: lams_tool_lamc/src/java/org/lamsfoundation/lams/tool/mc/model/McSession.java
===================================================================
diff -u -rb0523a63bf120c687d9b50748f4221f8d54aefea -r6ce1b949a486cdf029290566855cb29626ed0818
--- lams_tool_lamc/src/java/org/lamsfoundation/lams/tool/mc/model/McSession.java	(.../McSession.java)	(revision b0523a63bf120c687d9b50748f4221f8d54aefea)
+++ lams_tool_lamc/src/java/org/lamsfoundation/lams/tool/mc/model/McSession.java	(.../McSession.java)	(revision 6ce1b949a486cdf029290566855cb29626ed0818)
@@ -25,7 +25,9 @@
 import java.io.Serializable;
 import java.util.ArrayList;
 import java.util.Date;
+import java.util.HashSet;
 import java.util.List;
+import java.util.Set;
 
 import javax.persistence.CascadeType;
 import javax.persistence.Column;
@@ -83,51 +85,51 @@
     private org.lamsfoundation.lams.tool.mc.model.McContent mcContent;
 
     @OneToMany(mappedBy = "mcSession", cascade = CascadeType.ALL, orphanRemoval = true)
-    private List<McQueUsr> mcQueUsers;
+    private Set<McQueUsr> mcQueUsers = new HashSet<>();
 
     @ManyToOne(fetch = FetchType.LAZY, cascade = CascadeType.ALL)
     @JoinColumn(name = "mc_group_leader_uid")
     private McQueUsr groupLeader;
 
     /** full constructor */
     public McSession(Long mcSessionId, Date sessionStartDate, Date sessionEndDate, String sessionStatus,
-	    org.lamsfoundation.lams.tool.mc.model.McContent mcContent, List<McQueUsr> mcQueUsers) {
+	    org.lamsfoundation.lams.tool.mc.model.McContent mcContent, Set<McQueUsr> mcQueUsers) {
 	this.mcSessionId = mcSessionId;
 	this.sessionStartDate = sessionStartDate;
 	this.sessionEndDate = sessionEndDate;
 	this.sessionStatus = sessionStatus;
 	this.mcContent = mcContent;
-	this.mcQueUsers = mcQueUsers != null ? mcQueUsers : new ArrayList<>();
+	this.mcQueUsers = mcQueUsers != null ? mcQueUsers : new HashSet<>();
     }
 
     public McSession(Long mcSessionId, Date sessionStartDate, String sessionStatus, String session_name,
-	    org.lamsfoundation.lams.tool.mc.model.McContent mcContent, List<McQueUsr> mcQueUsers) {
+	    org.lamsfoundation.lams.tool.mc.model.McContent mcContent, Set<McQueUsr> mcQueUsers) {
 	this.mcSessionId = mcSessionId;
 	this.sessionStartDate = sessionStartDate;
 	this.sessionStatus = sessionStatus;
 	this.session_name = session_name;
 	this.mcContent = mcContent;
-	this.mcQueUsers = mcQueUsers != null ? mcQueUsers : new ArrayList<>();
+	this.mcQueUsers = mcQueUsers != null ? mcQueUsers : new HashSet<>();
     }
 
     public McSession(Long mcSessionId, Date sessionStartDate, String sessionStatus,
-	    org.lamsfoundation.lams.tool.mc.model.McContent mcContent, List<McQueUsr> mcQueUsers) {
+	    org.lamsfoundation.lams.tool.mc.model.McContent mcContent, Set<McQueUsr> mcQueUsers) {
 	this.mcSessionId = mcSessionId;
 	this.sessionStartDate = sessionStartDate;
 	this.sessionStatus = sessionStatus;
 	this.mcContent = mcContent;
-	this.mcQueUsers = mcQueUsers != null ? mcQueUsers : new ArrayList<>();
+	this.mcQueUsers = mcQueUsers != null ? mcQueUsers : new HashSet<>();
     }
 
     /** default constructor */
     public McSession() {
-	this.mcQueUsers = new ArrayList<>();
+	this.mcQueUsers = new HashSet<>();
     }
 
     /** minimal constructor */
-    public McSession(Long mcSessionId, List<McQueUsr> mcQueUsers) {
+    public McSession(Long mcSessionId, Set<McQueUsr> mcQueUsers) {
 	this.mcSessionId = mcSessionId;
-	this.mcQueUsers = mcQueUsers != null ? mcQueUsers : new ArrayList<>();
+	this.mcQueUsers = mcQueUsers != null ? mcQueUsers : new HashSet<>();
     }
 
     public Long getUid() {
@@ -208,15 +210,15 @@
      * @return Returns the mcQueUsers.
      */
 
-    public List<McQueUsr> getMcQueUsers() {
+    public Set<McQueUsr> getMcQueUsers() {
 	return this.mcQueUsers;
     }
 
     /**
      * @param mcQueUsers
      *            The mcQueUsers to set.
      */
-    public void setMcQueUsers(List<McQueUsr> mcQueUsers) {
+    public void setMcQueUsers(Set<McQueUsr> mcQueUsers) {
 	this.mcQueUsers = mcQueUsers;
     }
 
Index: lams_tool_lamc/src/java/org/lamsfoundation/lams/tool/mc/service/McService.java
===================================================================
diff -u -rb0523a63bf120c687d9b50748f4221f8d54aefea -r6ce1b949a486cdf029290566855cb29626ed0818
--- lams_tool_lamc/src/java/org/lamsfoundation/lams/tool/mc/service/McService.java	(.../McService.java)	(revision b0523a63bf120c687d9b50748f4221f8d54aefea)
+++ lams_tool_lamc/src/java/org/lamsfoundation/lams/tool/mc/service/McService.java	(.../McService.java)	(revision 6ce1b949a486cdf029290566855cb29626ed0818)
@@ -535,7 +535,7 @@
 	    mcSessionMarkDTO.setSessionId(session.getMcSessionId().toString());
 	    mcSessionMarkDTO.setSessionName(session.getSession_name().toString());
 
-	    List<McQueUsr> sessionUsers = session.getMcQueUsers();
+	    Set<McQueUsr> sessionUsers = session.getMcQueUsers();
 	    Iterator<McQueUsr> usersIterator = sessionUsers.iterator();
 
 	    Map<String, McUserMarkDTO> mapSessionUsersData = new TreeMap<String, McUserMarkDTO>(
@@ -842,7 +842,7 @@
 	Set<McSession> sessionList = content.getMcSessions();
 	for (McSession session : sessionList) {
 	    Long toolSessionId = session.getMcSessionId();
-	    List<McQueUsr> sessionUsers = session.getMcQueUsers();
+	    Set<McQueUsr> sessionUsers = session.getMcQueUsers();
 
 	    for (McQueUsr user : sessionUsers) {
 
@@ -1414,7 +1414,7 @@
 	if (!existsSession(toolSessionId)) {
 	    try {
 		McSession mcSession = new McSession(toolSessionId, new Date(System.currentTimeMillis()),
-			McSession.INCOMPLETE, toolSessionName, mcContent, new ArrayList<McQueUsr>());
+			McSession.INCOMPLETE, toolSessionName, mcContent, new HashSet<McQueUsr>());
 
 		mcSessionDAO.saveMcSession(mcSession);
 
Index: lams_tool_lamc/src/java/org/lamsfoundation/lams/tool/mc/web/controller/McLearningController.java
===================================================================
diff -u -rb0523a63bf120c687d9b50748f4221f8d54aefea -r6ce1b949a486cdf029290566855cb29626ed0818
--- lams_tool_lamc/src/java/org/lamsfoundation/lams/tool/mc/web/controller/McLearningController.java	(.../McLearningController.java)	(revision b0523a63bf120c687d9b50748f4221f8d54aefea)
+++ lams_tool_lamc/src/java/org/lamsfoundation/lams/tool/mc/web/controller/McLearningController.java	(.../McLearningController.java)	(revision 6ce1b949a486cdf029290566855cb29626ed0818)
@@ -29,6 +29,7 @@
 import java.util.LinkedList;
 import java.util.List;
 import java.util.Map;
+import java.util.Set;
 import java.util.TimeZone;
 import java.util.TreeMap;
 
@@ -241,7 +242,7 @@
 	    // forwards to the leaderSelection page
 	    if (groupLeader == null && !mode.equals(ToolAccessMode.TEACHER.toString())) {
 
-		List<McQueUsr> groupUsers = mcSession.getMcQueUsers();// mcService.getUsersBySession(new
+		Set<McQueUsr> groupUsers = mcSession.getMcQueUsers();// mcService.getUsersBySession(new
 								      // Long(toolSessionID).longValue());
 		request.setAttribute(McAppConstants.ATTR_GROUP_USERS, groupUsers);
 		request.setAttribute(McAppConstants.TOOL_SESSION_ID, toolSessionID);