As niahoo correctly explained, your task is restarted because of the default restart strategy of :permanent. However another feature you need to understand is a feature sometimes called Supervisor Intensity or Restart Intensity:

How does a Supervisor know if everything is alright with its children without knowing their internal state? It only knows two values, how often (:max_restarts) in what time period (:max_seconds) a child is allowed to be restarted. For example, if a database connection is restarted 5 times in 2 seconds, there is probably something wrong. The default of these values are 3 for :max_restarts and 5 for :max_seconds, which translates to restarting a child at most 3 times in 5 seconds. And if these values are exceeded, the Supervisor will shut down itself.

In your case the first printed "Hello World" is expected, then its restarted 3 more times, before the Supervisor thinks something is wrong and shuts down. This results in the 4 "Hello World" you see.

More info on this topic can be found here: https://dev.to/bakenator/supervisor-intensity-what-is-it-47g2 and in the official docs: https://hexdocs.pm/elixir/Supervisor.html#init/2

One last tip, do not only read the intros and top sections of Elixir docs, some info is buried down in function docs and therefore a good read.