Membership fee doubled when fixed contribution option set to "no thank you"

Discovered while working on CRM-14354. Reproduced on drupal.demo.civicrm.org. To reproduce the error:

1) Create a contribution page that allows recurring memberships.
2) On the Memberships tab, check "Separate Membership Payment."
3) On the Amounts tab, check "Contribution Amounts section enabled" and add a fixed contribution option.
4) Sign up for a new membership using the front-end form. Select a membership and select "no thank you" for the additional contribution. (The error seems to occur whether or not "Please renew my membership automatically" is checked.)

On the confirmation screen, two line items appear: the membership fee and the additional contribution amount. Both are set to the price of the membership fee. Additionally, two contribution records are created.