diff --git a/cameleer-server-app/src/main/resources/db/migration/V13__alert_instances_open_unique.sql b/cameleer-server-app/src/main/resources/db/migration/V13__alert_instances_open_unique.sql
new file mode 100644
index 00000000..9881f9a1
--- /dev/null
+++ b/cameleer-server-app/src/main/resources/db/migration/V13__alert_instances_open_unique.sql
@@ -0,0 +1,7 @@
+-- V13 — Unique partial index: at most one open alert_instance per rule
+-- Prevents duplicate FIRING rows in multi-replica deployments.
+-- The Java save() path catches DuplicateKeyException and log-and-skips the losing insert.
+CREATE UNIQUE INDEX alert_instances_open_rule_uq
+  ON alert_instances (rule_id)
+  WHERE rule_id IS NOT NULL
+    AND state IN ('PENDING','FIRING','ACKNOWLEDGED');