Why are the Chinese playing the Chinese??

In table tennis for the gold?
I know it’s not horse related, but does anyone know?

They likely had multiple qualifiers. Just like there can be multiple individuals from one country swimming in the finals of a swimming event.

The USA played the USA for gold in Beach Volleyball a couple of times I’m pretty sure.
Some sports allow multiple teams/couples from same country to enter. Makes sense in sports like Table a Tennis and Beach Volleyball where certain countries are particularly strong and deep.
They used to say in the 80s and 90s that if the Germans were allowed to enter three separate teams in Dressage that they would have swept the medals.