diff --git a/admin/cron.php b/admin/cron.php
index b6e6a81b91d..bf078167e73 100644
--- a/admin/cron.php
+++ b/admin/cron.php
@@ -99,19 +99,6 @@
         }
         flush();
     
-    
-        /// Delete duplicate enrolments (don't know what causes these yet - expired sessions?)
-    
-        if ($users = get_records_select("user_students", "userid > 0 GROUP BY course, userid ".
-                                        "HAVING count(*) > 1", "", "max(id) as id, userid, course ,count(*)")) {
-            foreach ($users as $user) {
-                delete_records_select("user_students", "userid = '$user->userid' ".
-                                     "AND course = '$user->course' AND id <> '$user->id'");
-            }
-        }
-        flush();
-    
-    
         /// Delete old logs to save space (this might need a timer to slow it down...)
     
         if (!empty($CFG->loglifetime)) {  // value in days
diff --git a/lib/db/mysql.php b/lib/db/mysql.php
index 20765236992..b570656c7e6 100644
--- a/lib/db/mysql.php
+++ b/lib/db/mysql.php
@@ -976,6 +976,35 @@ function main_upgrade($oldversion=0) {
         execute_sql("ALTER TABLE `{$CFG->prefix}user_teachers` ADD INDEX enrol (enrol);");
     }
     
+    if ($oldversion < 2004112400) {
+        /// Delete duplicate enrolments 
+        /// and then tell the database course,userid is a unique combination
+        if ($users = get_records_select("user_students", "userid > 0 GROUP BY course, userid ".
+                                        "HAVING count(*) > 1", "", "max(id) as id, userid, course ,count(*)")) {
+            foreach ($users as $user) {
+                delete_records_select("user_students", "userid = '$user->userid' ".
+                                     "AND course = '$user->course' AND id <> '$user->id'");
+            }
+        }
+        flush();
+        
+        modify_database('','ALTER TABLE prefix_user_students DROP INDEX courseuserid;');
+        modify_database('','ALTER TABLE prefix_user_students ADD UNIQUE INDEX courseuserid(course,userid);');        
+
+        /// Delete duplicate teacher enrolments 
+        /// and then tell the database course,userid is a unique combination
+        if ($users = get_records_select("user_teachers", "userid > 0 GROUP BY course, userid ".
+                                        "HAVING count(*) > 1", "", "max(id) as id, userid, course ,count(*)")) {
+            foreach ($users as $user) {
+                delete_records_select("user_teachers", "userid = '$user->userid' ".
+                                     "AND course = '$user->course' AND id <> '$user->id'");
+            }
+        }
+        flush();
+        modify_database('','ALTER TABLE prefix_user_teachers DROP INDEX courseuserid;');
+        modify_database('','ALTER TABLE prefix_user_teachers ADD UNIQUE INDEX courseuserid(course,userid);');        
+    } 
+       
     return $result;
 
 }
diff --git a/lib/db/mysql.sql b/lib/db/mysql.sql
index e85a028e280..b511c12d98d 100644
--- a/lib/db/mysql.sql
+++ b/lib/db/mysql.sql
@@ -431,7 +431,7 @@ CREATE TABLE `prefix_user_students` (
   `enrol` varchar(20) NOT NULL default '',  
   PRIMARY KEY  (`id`),
   UNIQUE KEY `id` (`id`),
-  KEY `courseuserid` (course,userid),
+  UNIQUE KEY `courseuserid` (course,userid),
   KEY `userid` (userid),
   KEY `enrol` (enrol),
   KEY `timeaccess` (timeaccess)
@@ -455,7 +455,7 @@ CREATE TABLE `prefix_user_teachers` (
   `timeaccess` int(10) unsigned NOT NULL default '0',
   PRIMARY KEY  (`id`),
   UNIQUE KEY `id` (`id`),
-  KEY `courseuserid` (course,userid),
+  KEY UNIQUE `courseuserid` (course,userid),
   KEY `userid` (userid)
 ) TYPE=MyISAM COMMENT='One record per teacher per course';
 
diff --git a/lib/db/postgres7.php b/lib/db/postgres7.php
index 3cc2ad51454..e2e454322ba 100644
--- a/lib/db/postgres7.php
+++ b/lib/db/postgres7.php
@@ -722,6 +722,43 @@ function main_upgrade($oldversion=0) {
         execute_sql("UPDATE {$CFG->prefix}log_display SET field='firstname||\' \'||lastname' WHERE module='course' AND action='user report' AND mtable='user'");
     }
 
+    if ($oldversion < 2004112400) {
+
+        /// Delete duplicate enrolments 
+        /// and then tell the database course,userid is a unique combination
+        if ($users = get_records_select("user_students", "userid > 0 GROUP BY course, userid ".
+                                        "HAVING count(*) > 1", "", "max(id) as id, userid, course ,count(*)")) {
+            foreach ($users as $user) {
+                delete_records_select("user_students", "userid = '$user->userid' ".
+                                     "AND course = '$user->course' AND id <> '$user->id'");
+            }
+        }
+        flush();
+
+	// drop some indexes quietly -- they may or may not exist depending on what version 
+	// the user upgrades from 
+        execute_sql("DROP INDEX {$CFG->prefix}user_students_courseuserid_idx ", false);
+	execute_sql("DROP INDEX {$CFG->prefix}user_students_courseuserid_uk  ", false);        
+        modify_database('','CREATE UNIQUE INDEX prefix_user_students_courseuserid_uk ON prefix_user_students (course,userid);');        
+
+        /// Delete duplicate teacher enrolments 
+        /// and then tell the database course,userid is a unique combination
+        if ($users = get_records_select("user_teachers", "userid > 0 GROUP BY course, userid ".
+                                        "HAVING count(*) > 1", "", "max(id) as id, userid, course ,count(*)")) {
+            foreach ($users as $user) {
+                delete_records_select("user_teachers", "userid = '$user->userid' ".
+                                     "AND course = '$user->course' AND id <> '$user->id'");
+            }
+        }
+        flush();
+
+	// drop some indexes quietly -- they may or may not exist depending on what version 
+	// the user upgrades from 
+        execute_sql("DROP INDEX {$CFG->prefix}user_teachers_courseuserid_idx ", false);
+	execute_sql("DROP INDEX {$CFG->prefix}user_teachers_courseuserid_uk  ", false);
+        modify_database('','CREATE UNIQUE INDEX prefix_user_teachers_courseuserid_uk ON prefix_user_teachers (course,userid);');        
+    } 
+    
     return $result;
 }
 
diff --git a/lib/db/postgres7.sql b/lib/db/postgres7.sql
index 05da57a4715..824de0bf3f9 100644
--- a/lib/db/postgres7.sql
+++ b/lib/db/postgres7.sql
@@ -297,7 +297,7 @@ CREATE TABLE prefix_user_students (
    enrol varchar (20) NOT NULL default ''
 );
 
-CREATE INDEX prefix_user_students_courseuserid_idx ON prefix_user_students (course,userid);
+CREATE UNIQUE INDEX prefix_user_students_courseuserid_uk ON prefix_user_students (course,userid);
 CREATE INDEX prefix_user_students_userid_idx ON prefix_user_students (userid);
 CREATE INDEX prefix_user_students_enrol_idx ON prefix_user_students (enrol);
 
@@ -315,7 +315,7 @@ CREATE TABLE prefix_user_teachers (
    enrol varchar (20) NOT NULL default ''
 );
 
-CREATE INDEX prefix_user_teachers_courseuserid_idx ON prefix_user_teachers (course,userid);
+CREATE UNIQUE INDEX prefix_user_teachers_courseuserid_uk ON prefix_user_teachers (course,userid);
 CREATE INDEX prefix_user_teachers_userid_idx ON prefix_user_teachers (userid);
 CREATE INDEX prefix_user_teachers_enrol_idx ON prefix_user_teachers (enrol);
 
diff --git a/version.php b/version.php
index 15e517693c9..47286317b13 100644
--- a/version.php
+++ b/version.php
@@ -6,7 +6,7 @@
 // This is compared against the values stored in the database to determine 
 // whether upgrades should be performed (see lib/db/*.php)
 
-   $version = 2004112300;  // YYYYMMDD = date of first major branch release 1.4
+   $version = 2004112400;  // YYYYMMDD = date of first major branch release 1.4
                            //       XY = increments within a single day
 
    $release = '1.5 UNSTABLE DEVELOPMENT';    // Human-friendly version name