[Feature Request] Warn when opening same notebook twice

When you (usually, by accident) open the same notebook twice, an awkward situation may arise where changes can get lost. That is why an editor like `vim` will warn you when you open a file for editing that is already open in another session.

Would it be possible to add such a warning mechanism to the Jupyter Notebook Editor?

The **Notebook changed** warning (with Reload/Cancel/Overwrite options) is helpful, but it may come too late. Once, you end up in that situation, you may already have a merge problem for the changes made in the two edit sessions. It would be better to warn earlier, to prevent this situation.