Usually, when an issue is edited, an e-mail notification is sent to everyone involved with that issue.
When editing an issue with the Structure widget, the changes are saved on the server and the e-mail notification is sent when you:
So if you switch from editing one field of an issue to editing another field of the same issue immediately, no update will have happened and no mail will have been sent yet.
If you need to change several fields of an issue and avoid multiple e-mails being sent, edit one field then navigate to the next field. Only hit Done or Enter when you have finished editing all fields.
To switch from editing one field to editing another field, you can:
So if you edit a field, click Done, then edit another field - that's two edits and there will be two notifications. If you edit a field, then edit another field, and only then click Done - that's one edit and one notification.