Email marketing

I need a system that allows me to enter my growing database of email addresses and names that automatically sends emails/newsletters every few weeks. Ideally the system will be able to remove names of consequent subscibers so that those who join up will not be re-emailed and the system would not email an address twice at the same time if it was in the system as a duplicate. Current database is only about 500 but will be growing by approx 100-300 weekly. I have a number of online businesses so there will be a LOT of REGULAR work to the right people. Thank you