// code placeholder

var bulkOps = new List<WriteModel<Student>>();
var bulkOps = new List<WriteModel<Student>>();
foreach (var student in students) 
{ 
var newId = Guid.NewGuid(); 
var update = Builders<Student>.Update 
  .Set(a => a.FirstName, student.FirstName) 
  .Set(a => a.LastName, student.LastName) 
  .Set(a => a.SomeField1, student.SomeField1) 
  .Set(a => a.LastModifiedDate, student.LastModifiedDate) 
  .SetOnInsert(a => a.PostedDate, student.PostedDate) 
  .SetOnInsert(a => a.Id, newId); 
var upsertOne = new UpdateOneModel<Student>(Builders<Student>.Filter.Where(x => x.FirstName == student.FirstName && x.LastName == student.LastName), update) { IsUpsert = true }; 
bulkOps.Add(upsertOne);  

//-- if executing individually, upsert works just fine. // var result = await Collection.UpdateOneAsync(x => x.FirstName == student.FirstName && x.LastName == student.LastName, update, new UpdateOptions { IsUpsert = true });

}

//-- This will throw duplicate exceptionvar result = await Collection.BulkWriteAsync(bulkOps);