I really can't add much to what Penny says. We like her, try to have at least one new port on each cruise but we have done the same Western Caribbean cruise a few times only because we live close to the ports those sail from and its less expensive for us to cruise on those ships.
However, saying that, we also are branching out from the Caribbean and have done a Mediterranean cruise, and an Alaska cruise and have plans to do a Western Mexico and a Hawaiian cruise (of course those are dependent on finances.. So if you had the money you certainly could take cruises to different ports each time and easily hit platinum status without doing too many duplicates. Or you can just enjoy the same ports many times. After all you are only visiting these places for about 6 hours per cruise. How much can you experience in a country in 6 hours? So there is plenty of opportunity to see new things.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007