A contact can be enter an automation more multiple times if the automation has multiple triggers and the contact meets the conditions for those triggers. This results in duplicate entries for your automation, and each entry will proceed through each action in your workflow.
For example, if a contact enters into your automation through two different triggers simultaneously, they will enter the automation twice. If each contact entry moves through a "send email" action, the automation will send the email twice to the same contact. This is because they entered your automation twice.
If a contact is part-way through or has completed an automation and reenters it, they will proceed through each action. If they move through a "Send email" action, that email will resend to them. To prevent this scenario from happening, read how do I prevent contacts from being added to my automation twice?
Learn more about automation start triggers and how to trigger automations in ActiveCampaign.