Automation emails are given a unique campaign ID that opens are associated with:
Once an automation email is opened by a contact, the open for that campaign ID is logged in our system for that contact.
If the contact enters your automation again and is resent the email but does not open it, the open for that campagin ID will still associated with that contact and they will always meet conditions in your automation for having opened that email.
For example, let's say Contact A enters an automation called "New Sign Ups" and is sent an email called "Thank you for your interest." That automation email has a campaign ID of 60. Contact A opens the email with the ID of 60 and that open action is logged in our system. They then move down the "If/Else" path in that automation for having opened that email.
Fast forward one month. Contact A reenters your automation and is resent the "Thank you for your interest" email that has the campagin ID of 60. They do not open the email this time around, but still go down the path in the "If/Else" statement for having opened that email. The reason why is because Contact A already opened the email with the ID of 60 one month ago. Because of this open action, they will always meet the conditions in your automation for having opened that email.