Each recipient of a webform submission is sent an individual email. For contact forms where multiple folks are collectively responsible for ensuring the submitter is responded to [only once], having a single thread -- where Reply-all can be used -- is extremely helpful.
I believe this is how Webform used to work, including all recipients in the To: field of a single message...
Can we add an option to the "E-mail to address" to send individually or together? I will write the patch if the functionality [does not exist and] is welcome.