MongoDB / Mongoose - aggreagte to identifing duplicated referenced values

How can I find all the users who reviewed a tour more than once in my collection at once?

(user and tour are referenced)

(schema)

  tour: {
    type: mongoose.Schema.ObjectId,
    ref: 'Tour',
    required: [true, 'The tour must be provided'],
  },
  user: {
    type: mongoose.Schema.ObjectId,
    ref: 'User',
    required: [true, 'The reviewer is required'],
  }

(document sample)

{
  "_id": "61f562af83e29c4f414df5c6",
  "review": "test review",
  "rating": 5,
  "createdAt": "2022-01-29T15:51:16.437Z",
  "tour": ObjectId("61f2a959c55314ec0a6e0e66"),
  "user": ObjectId("5c8a211f2f8fb814b56fa188"),
  "id": "61f562af83e29c4f414df5c6"
}