rake db:migrateでerrorが発生します。

Ruby on Railsで自分のappに、お気に入りの機能を追加しようとclips modelを作り、
マイグレーションファイルを編集してrake db:migrateを実行したのですが、

StandardError: An error has occurred, this and all later migrations canceled:　Index name 'index_clips_on_user_id' on table 'clips' already exists

　というerrorが発生してしまいます。rake db:migrateをできるようにするにはどうしたらよいでしょうか。

schema.rb

ActiveRecord::Schema.define(version: 20170318163142) do

  create_table "comments", force: :cascade do |t|
    t.text     "content"
    t.integer  "message_id"
    t.integer  "user_id"
    t.datetime "created_at", null: false
    t.datetime "updated_at", null: false
    t.index ["message_id"], name: "index_comments_on_message_id"
    t.index ["user_id"], name: "index_comments_on_user_id"
  end

  create_table "messages", force: :cascade do |t|
    t.string   "title"
    t.text     "description"
    t.datetime "created_at",  null: false
    t.datetime "updated_at",  null: false
    t.integer  "user_id"
  end

  create_table "relationships", force: :cascade do |t|
    t.integer  "follower_id"
    t.integer  "following_id"
    t.datetime "created_at",   null: false
    t.datetime "updated_at",   null: false
    t.index ["follower_id", "following_id"], name: "index_relationships_on_follower_id_and_following_id", unique: true
    t.index ["follower_id"], name: "index_relationships_on_follower_id"
    t.index ["following_id"], name: "index_relationships_on_following_id"
  end

  create_table "users", force: :cascade do |t|
    t.string   "email",                  default: "", null: false
    t.string   "encrypted_password",     default: "", null: false
    t.string   "reset_password_token"
    t.datetime "reset_password_sent_at"
    t.datetime "remember_created_at"
    t.integer  "sign_in_count",          default: 0,  null: false
    t.datetime "current_sign_in_at"
    t.datetime "last_sign_in_at"
    t.string   "current_sign_in_ip"
    t.string   "last_sign_in_ip"
    t.datetime "created_at",                          null: false
    t.datetime "updated_at",                          null: false
    t.string   "username"
    t.index ["email"], name: "index_users_on_email", unique: true
    t.index ["reset_password_token"], name: "index_users_on_reset_password_token", unique: true
    t.index ["username"], name: "index_users_on_username", unique: true
  end

end

　　
create_clips.rb

class CreateClips < ActiveRecord::Migration[5.0]
  def change
    create_table :clips do |t|
      t.references :user, null:false
      t.references :message, null:false
      t.timestamps null: false
    end

    add_index :clips, :user_id
    add_index :clips, :message_id
  end
end