Tipping even after a service charge is added on your bill?

I had this discussion with a friend recently she said that if they take 15-18% then she doesn't add on a gratuity since they took it already as a service charge. I always thought you tip anyways. That's what I've always done but ive heard many people say that since they add 18 % on your bill tipping an additional let's say even only 20% would make it 38% in actuality. So what do you think is the correct thing to do. Is it faire that since they already charged a service charge to not tip or is it wrong to not tip in addition to the service charge? I'll admit that when I was young the tip average was 10% and that was a good tip it seems that today it's between 15%-20% ive always tipped at least 35% anyways forever and always but curious what others think?

Ps. Since I can't edit this poll once I post I want to say that I know not all places take a service charge and of they do it will say service charge on your bill with an amount which you can calculate to estimate how much percentage they took for the tip.