diff --git a/schema/20181116.sql b/schema/20181116.sql
index cb97acf..48baa8f 100644
--- a/schema/20181116.sql
+++ b/schema/20181116.sql
@@ -57,7 +57,7 @@ CREATE TABLE notifications (
 );
 
 CREATE TABLE active_checks (
-    check_id bigint NOT NULL references checks(id) on delete cascade,
+    check_id bigint NOT NULL unique references checks(id) on delete cascade,
     cmdline text[] NOT NULL,
     next_time timestamp with time zone DEFAULT now() NOT NULL,
     states integer[] DEFAULT ARRAY[0] NOT NULL,