Free membership results in two activities and email receipts

Steps to recreate:

1. create a new membership type
2. give it a fee of zero
3. create a membership contribution page
4. turn off all payment settings
5. turn on membership settings
6. allow only the free membership
7. make membership a requirement for the form
8. set up a receipt email

Then on completing the contribution form you will receive the receipt email twice and two contribution activities will be recorded on the contact.