Avoid conflict and data loss in draft email opened on two different computers by the same user

If a user starts writing an email in webmail/inotes/verse/mobile or in the Notes client and saves it as a draft but leave it still open (for example on a desktop PC) and then opens on another computer (e.g. a laptop) in webmail/inotes/verse or in the Notes client the same draft that is still open on the other device and continues writing and saves the email draft, too. A
Afterwards the user gets back to his desktop computer and continues writing in the draft that is still open and send this email, every change that he did on the other device - the laptop computer - will get lost without any notification.
The user should at least get a notification that the email draft is still open in another session and if he continues writing then the other draft will lose data. Or the user should be refused to open and write to the draft that is still open on another computer and he should get a notification that he can only continue to write on a new draft but not on the draft document that is still open in another session.
The user should never lose data because of editing a document that is open in different session by himself if he changes devices.