You can configure email to case to settings to customize the way incoming emails are processed. It doesn't appear as though the search for a user contact has to only find one result for the owner of a case. However, its not clear to me what kind of contact you're asking about: user/owner contact or customer contact.
From the documentation:
One option is to select a checkbox to allow case owners to automatically receive notifications of new emails for their existing cases. You can define your email routing address settings after you add and verify your email routing addresses for Email-to-Case and On-Demand Email-to-Case.
Email-to-Case source only: Select this checkbox to automatically assign a task to the case owner when an email is submitted as a case. Assignment rules automatically assign owners to a case; however, if a case does not match assignment rule criteria, then the user in the Default Case Owner field on the Support Settings page is assigned to the case.
Origin fields auto-populate the case via the routing address settings when the routing address is included in either the To, CC, or BCC fields of an inbound email.
The above info came from these two reference pages:
Email-to-Case Settings and
Routing Address Settings for Email-to-Case and On-Demand Email-to-Case.
You might also find Jodie Miners blog post discussing email to cases titled
Salesforce and Email – a match made in hell of interest to you.