Duplicate data

[caveat1]

Hello,

I have an invoicing database, with a related customer table. There is also a mailing list-type table that is used to track marketing contacts and send out targeted mailings to customers or potential customers. A third table holds information on authors. Any of the people in the three tables could become a customer that would have an invoice.

Do I create duplicate entries in the customer table when people in either of the other tables buy something? If so, how would I keep their contact information up-to-date in both tables?

Does it make sense to create one big table with all the fields from all three tables?

In the other direction, customers could become people we want to put into a targeted mailing list in the mailing list table.

I'm not sure how to handle these situations. Is there a standard way to handle this kind of thing. Neither of these options seems like a very clean way to manage data. Any ideas would be welcome.