Two adjacent "To continue, you must log in" messages

1. Become logged out from Launchpad, for whatever reason (e.g. because you cleared your cookies).
2. In one window, open a page that requires login (such as <https://launchpad.net/products/schooltool/2006/+pots/schooltool/lt/170/+translate>).
3. Before the first page starts loading, in another window, open another page that requires login (such as <https://launchpad.net/products/launchpad/+filebug>).

What happens: One of the resulting login pages has two consecutive banners saying "To continue, you need to log in". On the other page, this text isn't present at all.

What should happen: The text should appear once and only once on every login page.

This is a side-effect of the way we handle alert messages, because we don't cruft up our URLs with tokens linking a particular message to a particular browsing path. However, we know that the login page should *always* contain the text "To continue, you need to log in", so it needn't be implemented as an alert message in the first place.