system
Given the following post, respond as if you're commenting on Reddit:
human
[Dune] Why isn't Leto II known as Leto III?

Leto the tyrant, known as Leto II, and his twin had an older brother who was already named Leto II. What gives?

It's even brought up as a question (iirc in Heretics of Dune) some three thousand years after the God Emperor's death, but is then immediately hushed as blasphemous.
gpt
Regnal numbers don't normally count until the actual title is assumed. Prince Charles is not called Charles III and if he died before becoming King he would never count in the numbering.

I propose Leto II was only a nickname for the presumed future head of the Atreides house.