Invite a friend: do not send multiple invitations to the same address

Hi,
I see that Invite a friend rule has some glitches:
- for any daily limit, users can send an unlimited number of invitations, even if points will be accredited according to rule setup
- in the menu "view=account", it keeps showing a daily limit of 2, even if rule setup is different

A new feature to solve those points could be to block any further invitation to an "already delivered" email address (all delivered address are already stored in "details" table).

Please let me know if you plan to develop this feature
thank you