Pending Approval mail is send everytime after trying to login just after this Pending Approval mail has been send to user.
So step by step:
1) User starts registration
2) Pending Approval mail is send to user
3) User does NOT confirm this Pending Approval mail
4) User tries to Login and get failure (_LOGIN_NOT_CONFIRMED)
5) User gets Pending Approval mail again!
- After few minutes -
6) User tries to Login and get failure (_LOGIN_NOT_CONFIRMED)
7) User gets Pending Approval mail again!!
As far as I know this is how it's ment to be. When a user doesn't confirms his registration and tries to login he gets the mail again. Just to make it clear.