Guild Wars 2

With alacrity your portal cooldown can up done before you use you old portal, and if you do use your portal (generally in the last 10-15 sec) you will get a fresh 70 sec cd without getting a 2nd one. (i tried to explain this as best as i could)

If you use the second portal, it will always reset the cooldown of portal.

If you don’t use the second portal, when the duration of the initial portal ends, the cooldown will be based off when you dropped the first portal.

Tl;dr: Everytime you place a portal (entrance or exit), the cooldown goes off.