[buileminh]

I found 2 problems:
1. I choose "YES" for "Only invited users can register" but I still can registered and validate my email without entering any invitation code.
2. If there is any problem that make invitation is sent duplicatedly to the same email, all the duplicated invitation are counted. This is not actually an error but it can be much better:
- If the email address is already registered with an existing member, no invitation will be sent and inviter should receive a notice about that
- If there are more than 1 invitations are sent to same emaill address by same member, only the last one are kept and counted, the others will be auto expired and deleted.