check to see if fieldmap object exists in SF?

Hello, I am working on creating fieldmaps between User accounts in Drupal and Contacts in Salesforce. One obstacle I am encountering is duplicating existing Contacts in Salesforce.

Ideally, one could compare the email address used in the Drupal user account registration with all emails used in Contacts in Salesforce. If a match was found, then the Drupal user account would be mapped to that Contact record. If no match was found, a new Contact could be happily created without fear of duplicating an existing Contact.

Does anyone have thoughts on where the best place to conduct this email verification (between new User accounts and existing Salesforce accounts) would be? And how to implement it - as an action?

Any advice appreciated. Thanks!