How does Tor resolve
.onion Domains? Are there also some central DNS servers in the Tor network that store these records? Is there a chance that two hidden services generate the same domain name?
It doesn't resolve them in the common sense - there is no DNS involved at all.
It looks up their introduction point on the hidden service directories - see Is it possible to look up the public key for a .onion-address?.
Then a rendezvous point is set up where the hidden service and the client meet - answers to How do onion addresses exactly work? have more details.
The hostname is just the (truncated) hash of the public key - it's highly unlikely that two services create the same hostname.