It's an interesting one. I assume that you have a new page called 'login', and you've placed the login control on it?
The problem is that the url rewriter places 4 'standard' pages into the page dictionary - which are login, register, terms and privacy. These represent the links for the standard skin objects.
Because you've added another page called login, that is why you're getting the duplicate page warning, even though you've only created one page.
To get around it in the short term, change the name of your login page - user-login or something would do. In the long term, I'll have a think about this problem and the implications, and see if I should remove the 'standard' link if it is overriden by a 'custom' page.
-Bruce