diff --git a/schema.sql b/schema.sql
index 893d56c..6cd83b3 100644
--- a/schema.sql
+++ b/schema.sql
@@ -26,5 +26,6 @@ create table if not exists tells (
     from_user text not null,
     to_user text not null,
     message text not null,
-    created timestamp not null default current_timestamp
+    created timestamp not null default current_timestamp,
+    unique (to_user, message)
 );