I don't think repeating a port, even multiple times, is a problem. There are always new things to do in port, or doing the same thing you did before and enjoyed is ok, too.
Some people like to focus on the ship, and even in that case, some like to try new ships/cruise lines, and some like to return to their favorite ship.
My cruises are usually divided into relaxing cruises vs sightseeing cruises. On relaxing cruises, which are usually to the Caribbean, I don't care if I've been to the ports before because the ship is more of a focus for me.
On sightseeing cruises, the ports have more importance to me. In that case, I like visiting new places. Itineraries often visit at least one port I've seen before, but that's ok. There are always new excursions to do. A couple of years ago, I went to Alaska for the second time, and had totally different experiences than the first time around.
As always, it's personal preference. Either way, you can't go wrong.