Automation triggers can be set to "Runs once" or "Runs multiple times." This setting is located in the trigger "Action Options" modal:
An automation trigger that is set to “Runs once” will only allow contacts to enter your automation if they meet the trigger conditions once. They will not be able to re-enter through that trigger. For example, if your automation has a trigger of “form is submitted” and is set to run once, contacts can only enter your automation through that trigger one time.
“Runs multiple times” will allow a contact to enter an automation through that specific trigger each time they fulfill the trigger conditions. For example, if your automation has a trigger of “Opens/Reads email” and is set to run multiple times, your contacts will enter the automation each time they open your email.
If you are using “Runs multiple times” with a “Subscribes to list” trigger, then contacts will only be re-added to your automation if they unsubscribe from your list and then re-subscribe to your list.