Odoo allows different users to modify the same register, while preserving the changes made by every one. I think it's a very well-designed functionality. If more than one modifies the same field, the last modification done prevails. Someone could see it as a problem, but the same thing would happen if they did it with 5 minutes of difference.

However, consider this scenario:
- User 1 and user 2 open the same order. It has a single line.
- User 1 adds several lines and updates.
- User 2 emails the order (he only sees one line, but it actually has several of them. He realizes it when it's already sent.

I think Odoo should raise a warning in this cases.