how to find duplicate records in mongo db query to use

We can first find the unique ids using

const data = await db.collection.aggregate([
  {
    $group: {
      _id: "$eventId",
      id: {
        "$first": "$_id"
      }
    }
  },
  {
    $group: {
      _id: null,
      uniqueIds: {
        $push: "$id"
      }
    }
  }
]);

And then we can make another query, which will find all the duplicate documents

db.collection.find({_id: {$nin: data.uniqueIds}})

This will find all the documents that are redundant.

Another way To find the event ids which are duplicated

db.collection.aggregate(
    {"$group" : { "_id": "$eventId", "count": { "$sum": 1 } } },
    {"$match": {"_id" :{ "$ne" : null } , "count" : {"$gt": 1} } }
)