What's changed
Duplicating a server creates a brand-new server with the same plan, game egg and startup settings, at no extra charge. It's a clean install - your world, plugins and other files are not copied over, so it's best for testing an update or configuration change before applying it to your live server.
You can have one active duplicate per server at a time. Cancel it when you're done to make another.
A server that needs its own manually-entered key to run - a CS:GO Steam token, for example - can't be duplicated, since that kind of key can only power one running server at a time.