Btw, I think you are mixing up a few topics here, so let’s see what you actually want.
Protocols are what computers use to communicate with each other. No protocols means no interaction between different computers/servers. Without protocols, none of the things you ask for can be possible.
Federated services don’t have single sign on. On the contrary, single sign on is a centralized service not a distributed one. To clarify that: I cannot log into lemmy.world with my feddit.de accout, same as I cannot log into hotmail with my gmail account. In both cases I log into my instance/provider and this allows me to communicate with people on other instances/providers. Federation is the process of sharing content between instances. SSO on the other hand is a centralized service that then communicates with other services to let you log into these other services. For example, I can log into my Google account and then use this to login to other sites. This only works because people trust Google. This would not work as a decentralized service with untrusted servers.
Duplication is used on federated services for a few reasons. First, it’s a kind of caching mechanism distributing the load. If someone posts something on one instance, it’s transferred only once to the other instances which then serve it to all their users. Without duplication, each individual view would have to be requested again from the original instance. The other advantage is that the admins of all the instances retain control over the content. If the other instance goes offline, users can still see “their” copy of the content. And if the other instance doesn’t moderate their content, the mods/admins of your instance can do that themselves.
So as you see, these concepts aren’t there just for fun, but for a purpose.