We hardly ever repeat cruises that sail to ports that we have already visited but we have sailed on the same ship more than once. We have been on the Celebrity Millennium three times: the first time we were on our Hong Kong to Singapore cruise, the second time we were in Alaska (our 2nd time in Alaska), and the third time was a cruise from Yokohama, Japan, to Vancouver, British Columbia, crossing the northern part of the Pacific Ocean and the International Date Line. We had three additional stops in Japan and one in Siberia, which was very interesting. During our voyage we had snow. Our crew members from Indonesia, Vietnam, Thailand, and the Philippines were very happy since they had never seen snow before.
We not only sailed on the Millennium three times but we stayed in the same cabin each time.
Another ship that we sailed on more than once was the Vision of the Seas with Royal Caribbean. We had sailed on that ship in 2001 on our second ever cruise and then again in 2014 to see Aruba, Bonaire, and Curacao. This ship also stopped in Key West and Grand Cayman. On our 2014 cruise, we had a really cool captain who used to play in a band and played rock music for the passengers in the evenings. We hoped he was our captain again when we sailed from Dubai to Venice on the same ship in 2017, but he was on vacation. We did not get a balcony cabin on any of these cruises because the ship is a small one and the balcony cabins were quite a bit more expensive than the ocean view.
We have sailed on the Crown Princess twice. For some reason I didn't like that ship we first sailed on it and less the second time but we have booked two more cruises on it, including an Atlantic crossing. I think the first time was because I got sick and was confined to our cabin and then after being cleared, I still had to go to the medical center and get clearance before going ashore in Malta, almost missing our tour. On our California Coastal cruise in 2015, the ship was getting ready to go into dry dock and there was a shortage of towels, washcloths, etc. Our cabin attendant had been with the cruise line many years and I don't think he was as much service-oriented as some of the ones who were trying to make a good impression. Our other future cruise on that ship is visiting some Caribbean islands that we haven't been to yet.
As you can probably tell, we choose destinations instead of cruise lines when we make a decision to cruise. Some cruise lines have better voyages to where we want to go than others.