When an existing member registers, they get a warning telling them they are already a member and are emailed a password so they can login to update their details.
If a customer who has previously enquired, or joined your mailing list, or purchased something from a shopping cart, or participated in a forum, then the database considers they are already a member based on uniqueness of their email address.
We have a policy of ensuring that only one member record exists per customer. This is to ensure that in the long run they can particpate in many facits of your website without requiring several different logins. It also ensures we can tie all their different histories to one central account.
The dilema is when a customer registers for the first time, we can trust that all their information is valid. However, if they register a second time, then we cannot trust that the information supplied is more valid than the information we already have on file, since because they have not logged in, then we don't know that the person submiting information is not a robot/spider/hacker. So the only option is to let the customer know what their password is (via email), and allow them to login and enter their profile that way
However, whenever a member reregisters, their mailing list subscription status will be updated if they were previously unsubscribed, as for many people this is the only reason they are registering.
Why can't members pick their own passwords?
Many of our webmasters just want members to register, not to login. It is also our belief that we make the registration easier by not providing questions for users to pick their own passwords, and ensure that a members email address is validated by sending them their password.
If a member really wants to pick their own password, then they can login to change their password. Please provide comments to your users in the appropriate places to explain this option.