How?
I'm re-watching the series, and today I watched The Two Towers, but how did Frodo know that Gollum's other name was Smegol?

oh. this is a tough question to answer if you haven't read the Hobbit. Cuz if you have then I could explain it in a heartbeat. But if you haven't... so have you read the Hobbit?

I've read The Hobbit like a million times.

okay good. Remember how Bilbo met Gollum and that's how he got the Ring? well Gollum must have revealed the Smeagol side of himself during that time. And then when Bilbo told Frodo about how he got the Ring... either that or when Gandalf told Frodo about the history of the Ring, he might have let Frodo know that Gollum was originally a Hobbit named Smeagol and all that...