Yes. Online Express will de-duplicate the email addresses from the mailing.

De-duplication will occur in the following scenarios:
  • Two queries in the same email list each return the same constituent record 
  • Two separate constituent records (like husband and wife) have the same email address 
  • Organization constituent and employee relationship each have the same email address
  • One constituent record has same email address under two (or more) different email types
  • One constituent has registered for multiple guests for an event with the constituents same email on every guest record