Requester Auto Creation creating duplicate users

We must have user auto creation turned on to meet a business need.

However, we are seeing many instances where an existing user is mailing requests to helpdesk and SD8 ( 8.0.0 Build 8005) is creating a duplicate user.

This causes problems in our site based user groups and technicians whose role it is to respond to a given site's tickets as the auto created accounts have a null site.

I would like to understand why this may be happening.

Please explain the matching algorithm that (at least sometimes) is able to match the existing (AD generated) user, but not always. I had been assuming it was an email address match, but some of the duplicates in question have identical email addresses.

I am wondering if forwarded or cc/bcc emails may have an impact as well.

Finally is there a way to dig into the matching algorithm and adjust it?

Thanks,
/kdl