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 through that trigger one if they meet the conditions to do so. 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 when they submit your form. If they submit your form again, they will not re-enter the automation through that trigger.
“Runs multiple times” will allow a contact to enter an automation through that specific trigger more than once if they meet the trigger conditions. It should be noted that contacts can only enter an automation through a "multiple runs" trigger up to 10 times within a 24 hour period. For example, let's say you're using the "Opens/Reads email" trigger and it's set to run multiple times. Then a contact opens your email and enters your automation at 2 pm CST. They can re-enter your automation through that trigger nine more times until 2pm CST the following day. Once 24 hours has passed, they can re-enter your automation up to 10 more times through that start trigger within a new 24 hour window, and so on.
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.