var collName = 'somecoll';
var coll = db[collName];
-
coll.drop();
coll.insertOne({'val': 'abc'});
-
var pipeline = [
  {"$unset": [
    "_id",
  ]},

  {"$merge": {
    "into": collName,
    "on": "_id",
    "whenMatched": "fail",
    "whenNotMatched": "insert"
  }},     
];
-
for (let i=0; i<3; i++) {
  coll.aggregate(pipeline).next();
  print(new Date() + ' - Count: ' + coll.countDocuments({}) + ', Size: ' + coll.stats().totalSize);
}