Index: lams_common/src/java/org/lamsfoundation/lams/dbupdates/patch20200129.sql
===================================================================
diff -u
--- lams_common/src/java/org/lamsfoundation/lams/dbupdates/patch20200129.sql	(revision 0)
+++ lams_common/src/java/org/lamsfoundation/lams/dbupdates/patch20200129.sql	(revision be35a3f2bf2414f299ca74f4ea8c3e2555b16d8e)
@@ -0,0 +1,22 @@
+SET AUTOCOMMIT = 0;
+SET FOREIGN_KEY_CHECKS = 0;
+
+--  LDEV-4959 Prevent leader and non-leader from creating two GradebookUserLessons at the same time
+
+--Take care about potential duplicates. For this move all entries to tmp table first.
+CREATE TABLE tmp_table SELECT * FROM lams_gradebook_user_lesson;
+TRUNCATE TABLE lams_gradebook_user_lesson;
+--Change key to unique
+ALTER TABLE `lams_gradebook_user_lesson` 
+	DROP FOREIGN KEY `FK_lams_gradebook_user_lesson_1`,
+	DROP INDEX `lesson_id`;
+ALTER TABLE `lams_gradebook_user_lesson` 
+	ADD UNIQUE INDEX `lesson_id` (`lesson_id`,`user_id`),
+	ADD CONSTRAINT `FK_lams_gradebook_user_lesson_1` FOREIGN KEY (`lesson_id`) REFERENCES `lams_lesson` (`lesson_id`);
+--Move entries back to lams_gradebook_user_lesson
+INSERT IGNORE INTO lams_gradebook_user_lesson SELECT * from tmp_table;
+DROP TABLE tmp_table;
+
+COMMIT;
+SET AUTOCOMMIT = 1;
+SET FOREIGN_KEY_CHECKS = 1;
\ No newline at end of file
Index: lams_gradebook/src/java/org/lamsfoundation/lams/gradebook/service/GradebookService.java
===================================================================
diff -u -rab88b7ff6a0dbd05c91a29e135b11f9596674f4c -rbe35a3f2bf2414f299ca74f4ea8c3e2555b16d8e
--- lams_gradebook/src/java/org/lamsfoundation/lams/gradebook/service/GradebookService.java	(.../GradebookService.java)	(revision ab88b7ff6a0dbd05c91a29e135b11f9596674f4c)
+++ lams_gradebook/src/java/org/lamsfoundation/lams/gradebook/service/GradebookService.java	(.../GradebookService.java)	(revision be35a3f2bf2414f299ca74f4ea8c3e2555b16d8e)
@@ -836,9 +836,10 @@
 
 	    // Now update the lesson mark
 	    if (gradebookUserLesson == null) {
-		gradebookUserLesson = new GradebookUserLesson();
-		gradebookUserLesson.setLearner(learner);
-		gradebookUserLesson.setLesson(lesson);
+		gradebookUserLesson = new GradebookUserLesson(lesson, learner);
+		gradebookDAO.insertOrUpdate(gradebookUserLesson);
+		//flush the session to delay the same gradebookUserLesson from being inserted by another user (it  primarily targets situation of leader updating marks for all non-leaders in a group)
+		gradebookDAO.flush();
 	    }
 
 	    boolean isWeightedMarks = toolService.isWeightedMarks(lesson.getLearningDesign());