Shaquille O'Neal is one of the best players in the game, known for his amazing strength and inside prowess. His amazing basketball abilities have gotten him the nickname "Superman", comparing him to the fictional superhero. Shaquille O'Neal notably has a Superman tattoo, showing his affinity for the superhero.
Later on in Shaquille O'Neal's career, another big man got the nickname Superman: Dwight Howard. Dwight Howard notably wore a Superman costume during the 2008 Dunk Contest. That has been a cause of friction among the two in the past. Recently, Shaquille O'Neal came out and commented on Dwight Howard being called Superman.
Shaquille O'Neal was one of the most dominant centers in the league during his prime, and he deserved the nickname he got, as at times it seemed like he was unstoppable. At the same time, Dwight Howard was an extremely gifted athlete, and he definitely had an amazing inside game. Both were top-tier centers in their era. There have been plenty of players with similar nicknames in the past, so two people sharing a nickname shouldn't be a big deal.
Despite Dwight Howard's ascension as "Superman" during his career, Shaquille O'Neal came first. Due to that, Shaquille O'Neal used to be very critical of Dwight Howard. At the end of the day, it doesn't matter all that much: both deserved the nickname for their elite performances on the court.