diff --git a/lib/completionlib.php b/lib/completionlib.php
index 4c8467bd0b9..cce41d0d922 100644
--- a/lib/completionlib.php
+++ b/lib/completionlib.php
@@ -935,13 +935,20 @@ class completion_info {
     function internal_set_data($cm, $data) {
         global $USER, $SESSION, $DB;
 
-        if ($data->id) {
-            // Has real (nonzero) id meaning that a database row exists
-            $DB->update_record('course_modules_completion', $data);
-        } else {
+        $transaction = $DB->start_delegated_transaction();
+        if (!$data->id) {
+            // Check there isn't really a row
+            $data->id = $DB->get_field('course_modules_completion', 'id',
+                    array('coursemoduleid'=>$data->coursemoduleid, 'userid'=>$data->userid));
+        }
+        if (!$data->id) {
             // Didn't exist before, needs creating
             $data->id = $DB->insert_record('course_modules_completion', $data);
+        } else {
+            // Has real (nonzero) id meaning that a database row exists, update
+            $DB->update_record('course_modules_completion', $data);
         }
+        $transaction->allow_commit();
 
         if ($data->userid == $USER->id) {
             $SESSION->completioncache[$cm->course][$cm->id] = $data;
diff --git a/lib/db/install.xml b/lib/db/install.xml
index 27b3c1e5b19..c83310c103f 100644
--- a/lib/db/install.xml
+++ b/lib/db/install.xml
@@ -367,8 +367,8 @@
         <KEY NAME="primary" TYPE="primary" FIELDS="id"/>
       </KEYS>
       <INDEXES>
-        <INDEX NAME="coursemoduleid" UNIQUE="false" FIELDS="coursemoduleid" COMMENT="For quick access via course-module (e.g. when displaying course module settings page and we need to determine whether anyone has completed it)." NEXT="userid"/>
-        <INDEX NAME="userid" UNIQUE="false" FIELDS="userid" COMMENT="Index on user ID. Used when obtaining completion information for normal course page view." PREVIOUS="coursemoduleid"/>
+        <INDEX NAME="coursemoduleid" UNIQUE="false" FIELDS="coursemoduleid" COMMENT="For quick access via course-module (e.g. when displaying course module settings page and we need to determine whether anyone has completed it)." NEXT="userid-coursemoduleid"/>
+        <INDEX NAME="userid-coursemoduleid" UNIQUE="true" FIELDS="userid, coursemoduleid" PREVIOUS="coursemoduleid"/>
       </INDEXES>
     </TABLE>
     <TABLE NAME="course_sections" COMMENT="to define the sections for each course" PREVIOUS="course_modules_completion" NEXT="course_request">
diff --git a/lib/db/upgrade.php b/lib/db/upgrade.php
index b47810c0c2c..54bd086443e 100644
--- a/lib/db/upgrade.php
+++ b/lib/db/upgrade.php
@@ -6062,6 +6062,60 @@ WHERE gradeitemid IS NOT NULL AND grademax IS NOT NULL");
         upgrade_main_savepoint(true, 2011022100.01);
     }
 
+    if ($oldversion < 2011033003.09) {
+        // Completion system has issue in which possible duplicate rows are
+        // added to the course_modules_completion table. This change deletes
+        // the older version of duplicate rows and replaces an index with a
+        // unique one so it won't happen again.
+
+        // This would have been a single query but because MySQL is a PoS
+        // and can't do subqueries in DELETE, I have made it into two. The
+        // system is unlikely to run out of memory as only IDs are stored in
+        // the array.
+
+        // Find all rows cmc1 where there is another row cmc2 with the
+        // same user id and the same coursemoduleid, but a higher id (=> newer,
+        // meaning that cmc1 is an older row).
+        $rs = $DB->get_recordset_sql("
+SELECT DISTINCT
+    cmc1.id
+FROM
+    {course_modules_completion} cmc1
+    JOIN {course_modules_completion} cmc2
+        ON cmc2.userid = cmc1.userid
+        AND cmc2.coursemoduleid = cmc1.coursemoduleid
+        AND cmc2.id > cmc1.id");
+        $deleteids = array();
+        foreach ($rs as $row) {
+            $deleteids[] = $row->id;
+        }
+        $rs->close();
+        // Note: SELECT part performance tested on table with ~7m
+        // rows of which ~15k match, only took 30 seconds so probably okay.
+
+        // Delete all those rows
+        $DB->delete_records_list('course_modules_completion', 'id', $deleteids);
+
+        // Define index userid (not unique) to be dropped form course_modules_completion
+        $table = new xmldb_table('course_modules_completion');
+        $index = new xmldb_index('userid', XMLDB_INDEX_NOTUNIQUE, array('userid'));
+
+        // Conditionally launch drop index userid
+        if ($dbman->index_exists($table, $index)) {
+            $dbman->drop_index($table, $index);
+        }
+
+        // Define index userid-coursemoduleid (unique) to be added to course_modules_completion
+        $index = new xmldb_index('userid-coursemoduleid', XMLDB_INDEX_UNIQUE,
+                array('userid', 'coursemoduleid'));
+
+        // Conditionally launch add index userid-coursemoduleid
+        if (!$dbman->index_exists($table, $index)) {
+            $dbman->add_index($table, $index);
+        }
+
+        upgrade_main_savepoint(true, 2011033003.09);
+    }
 
     return true;
 }
diff --git a/lib/simpletest/testcompletionlib.php b/lib/simpletest/testcompletionlib.php
index 66fc9cb7e52..f4288fd9eb0 100644
--- a/lib/simpletest/testcompletionlib.php
+++ b/lib/simpletest/testcompletionlib.php
@@ -6,6 +6,7 @@ require_once($CFG->libdir.'/completionlib.php');
 
 global $DB;
 Mock::generate(get_class($DB), 'mock_database');
+Mock::generate('moodle_transaction', 'mock_transaction');
 
 Mock::generatePartial('completion_info','completion_cutdown',
     array('delete_all_state','get_tracked_users','update_state',
@@ -452,24 +453,40 @@ WHERE
     function test_internal_set_data() {
         global $DB,$SESSION;
 
-        $cm=(object)array('course'=>42,'id'=>13);
-        $c=new completion_info((object)array('id'=>42));
+        $cm = (object)array('course' => 42,'id' => 13);
+        $c = new completion_info((object)array('id' => 42));
 
         // 1) Test with new data
-        $data=(object)array('id'=>0,'userid'=>314159);
-        $DB->setReturnValueAt(0,'insert_record',4);
-        $DB->expectAt(0,'insert_record',array('course_modules_completion',$data));
-        $c->internal_set_data($cm,$data);
-        $this->assertEqual(4,$data->id);
-        $this->assertEqual(array(42=>array(13=>$data)),$SESSION->completioncache);
+        $data = (object)array('id'=>0, 'userid' => 314159, 'coursemoduleid' => 99);
+        $DB->setReturnValueAt(0, 'start_delegated_transaction', new mock_transaction());
+        $DB->setReturnValueAt(0, 'insert_record', 4);
+        $DB->expectAt(0, 'get_field', array('course_modules_completion', 'id',
+                array('coursemoduleid' => 99, 'userid' => 314159)));
+        $DB->expectAt(0, 'insert_record', array('course_modules_completion', $data));
+        $c->internal_set_data($cm, $data);
+        $this->assertEqual(4, $data->id);
+        $this->assertEqual(array(42 => array(13 => $data)), $SESSION->completioncache);
 
         // 2) Test with existing data and for different user (not cached)
         unset($SESSION->completioncache);
-        $d2=(object)array('id'=>7,'userid'=>17);
-        $DB->expectAt(0,'update_record',array('course_modules_completion',$d2));
-        $c->internal_set_data($cm,$d2);
+        $d2 = (object)array('id' => 7, 'userid' => 17, 'coursemoduleid' => 66);
+        $DB->setReturnValueAt(1, 'start_delegated_transaction', new mock_transaction());
+        $DB->expectAt(0,'update_record', array('course_modules_completion', $d2));
+        $c->internal_set_data($cm, $d2);
         $this->assertFalse(isset($SESSION->completioncache));
 
+        // 3) Test where it THINKS the data is new (from cache) but actually
+        // in the database it has been set since
+        // 1) Test with new data
+        $data = (object)array('id'=>0, 'userid' => 314159, 'coursemoduleid' => 99);
+        $DB->setReturnValueAt(2, 'start_delegated_transaction', new mock_transaction());
+        $DB->setReturnValueAt(1, 'get_field', 13);
+        $DB->expectAt(1, 'get_field', array('course_modules_completion', 'id',
+                array('coursemoduleid' => 99, 'userid' => 314159)));
+        $d3 = (object)array('id' => 13, 'userid' => 314159, 'coursemoduleid' => 99);
+        $DB->expectAt(1,'update_record', array('course_modules_completion', $d3));
+        $c->internal_set_data($cm, $data);
+
         $DB->tally();
     }
 
diff --git a/version.php b/version.php
index 06757b0563e..175573d1920 100644
--- a/version.php
+++ b/version.php
@@ -30,7 +30,7 @@
 defined('MOODLE_INTERNAL') || die();
 
 
-$version  = 2011033003.08;              // 20110330      = branching date YYYYMMDD - do not modify!
+$version  = 2011033003.09;              // 20110330      = branching date YYYYMMDD - do not modify!
                                         //         RR    = release version - do not change in weeklies
                                         //           .XX = incremental changes