diff --git a/classes/booking_rules/rules/rule_daysbefore.php b/classes/booking_rules/rules/rule_daysbefore.php
index 6fac0a149..83cb9ad77 100644
--- a/classes/booking_rules/rules/rule_daysbefore.php
+++ b/classes/booking_rules/rules/rule_daysbefore.php
@@ -355,6 +355,7 @@ public function get_records_for_execution(
 
         $condition->execute($sql, $params, $testmode, $nextruntime);
 
+        $sql->select = " DISTINCT " . $sql->select; // Required to eliminate potential duplication in case inoptimal query.
         $sqlstring = "SELECT $sql->select FROM $sql->from WHERE $sql->where";
 
         $records = $DB->get_records_sql($sqlstring, $params);