Webform emails threading
This isn't really a Drupal question. Go to https://mail.google.com/mail/#settings/general and look at the Conversation View. This sets whether emails of the same topic are grouped together. So they aren't grouped together based on the from: address but on the title. Just set it to Conversation view off and save the settings.

Alternatively, make the subject of the mail unique. For example by adding the name or email address to it, or a unique key.