Can a person be both a client and a server? (and more)

In an authoritative server setup, can a player be both the server and a client, or must there be a separate exe running for just the server? For example, can both isClient and isServer be returned true on the same instance?

Also, with an authoritative server, it is harder for things to become out of sync, since the server is taking everyone's inputs and simulating it in a single environment, right? Compared to a non-authoritative server where everyone's telling each other what they're doing and simulating it on each other's end?

Thanks