Duplicate places enhanced versions of Copy and Paste into the Finder. They behave similarly to those that are already built in, with the exception that they are a lot more powerful. With this you won't get failed copies on large folders, and you wont get any problems with permissions. It is also about 10% faster.
It will make an exact duplicate which perfectly passes the backup bouncer test, which is a test that determines whether a perfect copy of a file and all associated data is made, which many things (including the Finder's native copy and paste) fail.
What it will do is allow you to effectively copy and paste anything. The ultimate test is that you can even copy an entire volume with it, and the volume that you paste it onto will be bootable.