What happens if I edit a duplicate Hosts file?
This is a poorly chosen vocabulary since in a MacOS X file system a duplicate is a serious error meaning that the file-system is deeply corrupted.

The correct term which should have been used here is the term of copy.

If you don't have permission to edit /private/etc/hosts then make a local copy as follows:

cp /private/etc/hosts ~/hosts


If you really need to edit the /private/etc/hosts, I advise you to make first a backup local copy as above. And once you have edited and thoroughly verified it,
put it back into place with sudo to get the admin priviledges:

/usr/bin/sudo cp ~/hosts /private/etc/hosts