I'm not sure if I'm right but I thought that when I send a message then there is only one mail object in the system. Using getFolderRequest on Mailbox, I can see that there are two different items - normal and one in send (I've send email to myself). This is ok. The items has different id and msgId which is the same for both of them.

The problem is how can I do anything when I have this shared id - I would like to change the subject of a message (for both items - normally I don't want to send e-mail to myself ;) ). Is it possible at all?