外部キー同士の組合せの重複保存ができないようにUnique制約を追加します。
尚、作成するテーブルはcommentsテーブルとし、カラムである外部キーはuser_idとpost_idとします。

MigrationID_MigrationName.rb

class CreateComments < ActiveRecord::Migration[6.0]
  def change
    create_table :comments do |t|
      t.references :user, null: false, foreign_key: true
      t.references :post, null: false, foreign_key: true
      t.text :text, null: false
      t.timestamps
    end
    add_index :comments, [:user_id, :post_id], unique: true #user_id × post_id = 一意の値としてUnique制約を追加
  end
end

テーブルの設計が完了したら作成する。

$ bundle exec rake db:migrate

テーブルの作成とindexの確認をした後に、検証する。

$ rails db

mysql> show tables;
+-----------------------------+
| Tables_in_name_development |
+-----------------------------+
| comments                    |
    .
    .

mysql> show index from comments;
+----------+------------+---------------------------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table    | Non_unique | Key_name                              | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+----------+------------+---------------------------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| comments |          0 | PRIMARY                               |            1 | id          | A         |           1 |     NULL | NULL   |      | BTREE      |         |               |
| comments |          0 | index_comments_on_user_id_and_post_id |            1 | user_id     | A         |           1 |     NULL | NULL   |      | BTREE      |         |               |
| comments |          0 | index_comments_on_user_id_and_post_id |            2 | post_id     | A         |           1 |     NULL | NULL   |      | BTREE      |         |               |
| comments |          1 | index_comments_on_user_id             |            1 | user_id     | A         |           1 |     NULL | NULL   |      | BTREE      |         |               |
| comments |          1 | index_comments_on_post_id             |            1 | post_id     | A         |           1 |     NULL | NULL   |      | BTREE      |         |               |
+----------+------------+---------------------------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
5 rows in set (0.00 sec)

mysql> insert into comments (user_id, post_id) values(有効なUserID, 有効なPostID);
Query OK

mysql> insert into comments (user_id, post_id) values(上記同様, 上記同様);
ERROR 1062 (23000): Duplicate entry '5-17' for key 'index_comments_on_user_id_and_post_id'

Done.