If a service charge has been added to the bill, on top of the stated menu prices, then that is the tip. If the service was really great and you feel so moved as to tip some more, then sure, go ahead. If the service was "normal", then just pay the bill with the service charge.

I have to say that I rarely see this kind of service charge added to a bill UNLESS it's a large party. Restaurants will typically have some kind of notice on their menu that a grautity will be added automatically for parties of X or more. (Usually eight.)

This protects the restaurant from a situation where the person paying the bill for a large party stiffs the tip, either intentionally or not.

Having said that... the tip is still voluntary. I was in a large party, once, where the service was just horrible. The restaurant environment left a great deal to be desired, other words, it was not just the server's fault. We called the manager over and negotiated down the tip, for good reason. This was done after all in the group agreed that the service was way, way below par and that a poor tip was appropriate. (All involved were Americans, and were nice people who would normally tip properly.)