A contact can be added to an automation more than once if the automation has multiple triggers and they meet the conditions for multiple triggers. This results in duplicate entries for your automation and both entries will proceed through each step.
If a contact enters into your automation through two different triggers simultaneously and lands on a "send email" step, they will only be sent the email once.
If a contact is part-way through or has completed an automation and is then re-added through a different trigger, they will proceed through each step and automation emails will be resent to them. To prevent this scenario from happening, click here for a quick tip on what to do.
To find out more about automation start triggers, check out this guide.