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Index: moodle/backup/restorelib.php
--- moodle/backup/restorelib.php Base (1.283.2.94)
+++ moodle/backup/restorelib.php Locally Modified (Based On 1.283.2.94)
@@ -3240,10 +3240,11 @@
                                         //We have to recode the useridto field
                                         $user = backup_getid($restore->backup_unique_code,"user",$dbrec->useridto);
                                         if ($user) {
-                                            //echo "User ".$dbrec->useridto." to user ".$user->new_id."<br />";   //Debug
+                                         //   echo "User ".$dbrec->useridto." to user ".$user->new_id."<br />";   //Debug
                                             $dbrec->useridto = $user->new_id;
                                         }
-                                        //Check if the record doesn't exist in DB!
+                                        //Verify there is a new record for the user and then check if the record doesn't already exist in DB!
+                                        if ($dbrec->useridto>0 && $dbrec->useridfrom>0) { //prevents MDL-20526
                                         $exist = get_record('message','useridfrom',$dbrec->useridfrom,
                                                                       'useridto',  $dbrec->useridto,
                                                                       'timecreated',$dbrec->timecreated);
@@ -3254,6 +3255,7 @@
                                             //Duplicate. Do nothing
                                         }
                                     }
+                                    }
                                     //Do some output
                                     $counter++;
                                     if ($counter % 10 == 0) {