A set of viewers that drops duplicates

This code collects the people who viewed a post in a set. Predict the output without running the code.

Code

viewers = {"mina", "jun", "mina", "sora"}
print(len(viewers))
viewers.add("jun")
viewers.add("dami")
viewers.remove("mina")
print(len(viewers))
print("mina" in viewers, "dami" in viewers)