You "have seconds" or "go back for seconds."
Is it more a slang or formal language?Second helping is also used.
That is very good to know. Thank you for sharing these.It's neither slang nor particularly formal. It's just what some people choose to say.
I'm going back for more.
I'm going back for seconds.
I'm going back for a second helping.
Note that if you still want more after that, you go back for "thirds" or a "third helping", (then "fourth(s) etc).