Dublicate documents with transactions upsert

[alexov_inbox]

Hello. i use latest mongodb 5. Found intresting moment then update in parallel is not exists document with UPSERT by transaction i got dublicate documents

 using var session = DB.Client.StartSession();
 session.StartTransaction();

userId = 1;

        try
        {
          var u = Builders<Balance>.Update.Inc(f => f.Quantity, 5)
            .SetOnInsert(f => f.Id, Guid.NewGuid())

        var balanceAfter = Balances.FindOneAndUpdate<Balance>(session, c => c.UserId == 
         userId, u, new FindOneAndUpdateOptions<Balance>
        {
            IsUpsert = true,
            ReturnDocument = ReturnDocument.After
        });
            session.CommitTransaction();
        }
        catch (Exception e)
        {
            Console.WriteLine("Error MongoDB Transaction: " + e.Message);
            session.AbortTransaction();
        }

But if i run second time then DOCUMENT exists , all work current.

How to make the first option work like the second one?

I think the problem is that transactions with an upset do not understand that they will update the same document

[Asya_Kamsky]

There is no way for the system to know you didn’t intend to create a duplicate unless you have a unique index on appropriate field(s). This is not related to transactions and it’s described in the documentation for update and findAndModify.

Asya