The wording is a bit strange - technically all men (18-25) must register. When I tried to register, I was told I couldn't because I was already registered.

The Selective Service auto-registers people from various data sources.

But this puts me in a weird spot: I've never actually registered. I am registered. But I did not register - which is the requirement.

There are Kafka-esque parts of the US government where this distinction could matter.

You did register, you just didn't realize you did. Time honored tradition in the U.S.