How to choose the alias used in a menu I'm having a bit of a problem when using multiple aliases regarding which alias appears on a menu.

For example, I have create a page /node/1, give it an alias /page1 and add it into the navigation menu.

Browsing to the index page I can see the page in the navigation at /page1.

I also want the page to be available at another alias /page2 so I add this through the URL aliases functionality. Unfortunately this updates my menu so the link in the menu now points to the URL /page2 instead of /page1. 

If I have multiple aliases to a node, how does Drupal decide which is used in a menu and how can I change this
When multiple aliases are available, Drupal uses the most recently created (as determined by the pid column in the url_alias table).

I don't know an easy way to change this behavior.  You could try manually altering the pid value in the database.  Also consider if using a redirect is a better solution than aliasing.