diff --git a/db/newsgroup.sql b/db/newsgroup.sql
index e35f37a..e9edf5b 100644
--- a/db/newsgroup.sql
+++ b/db/newsgroup.sql
@@ -9,7 +9,7 @@ create table newsgroup (
 create table newsgroup_message (
     id           INTEGER PRIMARY KEY NOT NULL,
     newsgroup_id INTEGER NOT NULL,
-    message_id   TEXT NOT NULL,
+    message_id   TEXT NOT NULL UNIQUE,
     created_on   DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
     sender       TEXT NOT NULL,
     subject      TEXT NOT NULL,