Prevent username reusing

src/server/api/endpoints/username/available.ts

@@ -1,6 +1,6 @@
 import $ from 'cafy';
 import define from '../../define';
-import { Users } from '../../../../models';
+import { Users, UsedUsernames } from '../../../../models';
 
 export const meta = {
 	tags: ['users'],
@@ -21,7 +21,9 @@ export default define(meta, async (ps) => {
 		usernameLower: ps.username.toLowerCase()
 	});
 
+	const exist2 = await UsedUsernames.count({ username: ps.username.toLowerCase() });
+
 	return {
-		available: exist === 0
+		available: exist === 0 && exist2 === 0
 	};
 });