Is there a way to hide seen posts?

You can do that either on the website interface or the app on your profile settings.

On the app, go to the profile icon on the bottom nav bar > cog (top right) > accounts > your account > disable show read posts.

I've done this but it does not hide them for me. I keep seeing the same posts over and over

Edit: apparently it didn't save. I had to try a couple more times and it finally saved

I just checked Liftoff on my Testflight invite. Doesn’t seem to be a feature there yet.