Let me jump at this opportunity to unabashedly promote my own product.

Take a look at http://www.infusionwp.com.

There, you'll find a plugin that will handle all your Infusionsoft + Wordpress membership synchronization needs - and much more.

In general terms, the user login field in Wordpress is the only one guaranteed to be unique within a Wordpress installation.

Infusionsoft doesn't automatically take care of that for you. You'll have to do that manually - or tell your order and web forms to check for you. From time to time, you can merge contact records and eliminate duplicates.

A compromise would be to use a user's email address and use that as the user's Wordpress login. It would be easier to maintain everything synchronized, as few people are likely to share an email address.

Or get the plugin

Best,
Bob