Looking at your moderation history, you created a thread that focused primarily on events that had happened in the most recent episode, for which there was already a pinned thread for general and specific discussion of that episode.

The fact that you flagged it as spoilers indicated it was focusing on not just a particular character, but the events of the most recent episode. And that's how the discussion was going.

The way to avoid having a thread deleted as a duplicate of an existing episode thread would be to just post in the existing thread.

Note that your post was handled as "Moderator Discretion" which means that there was no penalty applied. Just direct the discussion to the pinned episode thread intended for that kind of discussion, and you'll be fine.