Using Set instead of List as Collection type for getUsersInSession to

prevent duplicates if the client uses Socket.IO and still sends the
**/online ping.

src/main/java/de/thm/arsnova/services/IUserService.java

@@ -39,7 +39,7 @@ public interface IUserService {
 
 	boolean isUserInSession(User user, String keyword);
 
-	List<User> getUsersInSession(String keyword);
+	Set<User> getUsersInSession(String keyword);
 
 	void addCurrentUserToSessionMap(String keyword);
 

src/main/java/de/thm/arsnova/services/UserService.java

@@ -7,7 +7,7 @@ import java.io.IOException;
 import java.io.ObjectInputStream;
 import java.io.ObjectOutputStream;
 import java.io.OutputStream;
-import java.util.ArrayList;
+import java.util.HashSet;
 import java.util.Hashtable;
 import java.util.List;
 import java.util.Map;
@@ -148,8 +148,8 @@ public class UserService implements IUserService, InitializingBean, DisposableBe
 	}
 
 	@Override
-	public List<User> getUsersInSession(String keyword) {
-		List<User> result = new ArrayList<User>();
+	public Set<User> getUsersInSession(String keyword) {
+		Set<User> result = new HashSet<User>();
 		for (Entry<User, String> e : user2session.entrySet()) {
 			if (e.getValue().equals(keyword)) {
 				result.add(e.getKey());

src/main/java/de/thm/arsnova/socket/ARSnovaSocketIOServer.java

@@ -265,7 +265,7 @@ public class ARSnovaSocketIOServer {
 		 * all connected clients and if send feedback, if user is in current
 		 * session
 		 */
-		List<User> users = userService.getUsersInSession(sessionKey);
+		Set<User> users = userService.getUsersInSession(sessionKey);
 
 		for (SocketIOClient c : server.getAllClients()) {
 			User u = userService.getUser2SocketId(c.getSessionId());