Drupal 7.16 started sending duplicate account activation emails
OK, not sure what was happening there! I changed the settings on admin/config/people/accounts and removed the need to confirm email address ("Require e-mail verification when a visitor creates an account.". 

This didn't work - still duplicates being sent out when activated.

Then I de-selected "Notify user when account is activated." (Under "Account Activation" tab) and this stopped them being sent.

Then I re-enabled "Notify user when account is activated." AND modified the email template.

Now it's only sending the one email again!

For some reason, this also fixed the rule emails that were getting duplicated... Beezaaar!

Any ideas?