diff --git a/federationsender/storage/postgres/blacklist_table.go b/federationsender/storage/postgres/blacklist_table.go
index f1db9fae..8de6feec 100644
--- a/federationsender/storage/postgres/blacklist_table.go
+++ b/federationsender/storage/postgres/blacklist_table.go
@@ -25,7 +25,8 @@ import (
 const blacklistSchema = `
 CREATE TABLE IF NOT EXISTS federationsender_blacklist (
     -- The blacklisted server name
-    server_name TEXT NOT NULL
+	server_name TEXT NOT NULL,
+	UNIQUE (server_name)
 );
 `
 
diff --git a/federationsender/storage/sqlite3/blacklist_table.go b/federationsender/storage/sqlite3/blacklist_table.go
index 3e302906..a14fe0c4 100644
--- a/federationsender/storage/sqlite3/blacklist_table.go
+++ b/federationsender/storage/sqlite3/blacklist_table.go
@@ -25,7 +25,8 @@ import (
 const blacklistSchema = `
 CREATE TABLE IF NOT EXISTS federationsender_blacklist (
     -- The blacklisted server name
-    server_name TEXT NOT NULL
+	server_name TEXT NOT NULL,
+	UNIQUE (server_name)
 );
 `