If I send emails to the same contact more than once, how Pipeline manage the contact?

If a user sends emails to the same contact multiple times,  the contact card will be moved to the stage according to the contact's last activity of the last campaign.

For example, if you send an email to Danny in Campaign-A and Danny Clicked the link in the email. 

Danny will be moved to the "Clicked" stage.

Later on, you decide to contact Danny again, the email reaches Danny but he hasn't opened the email. 

At this moment, Danny's contact card will be moved back to Delivered stage.

Each activity of the contact will be consolidated in the view of a open contact card, allowing users to view holistic contact history including sending dates, email activity, and the campaigns.