Don't eat another portion, especially if you're thinking "oh, it's healthy and it's going to balance out..." No, just don't eat again. Quit giving yourself excuses. If you are eating junk food for the moment then just eat it 'til you're done... You can start a new choice tomorrow by eating healthy. At least your body will thank you.
The best thing is not to eat the junk food eating something healthy after is pointless you already put a ton of calories into to your body