Can I diet during pregnancy if I am already overweight?

Yes. Yes so long as it is a healthy diet. I recommend weight watchers.
No. Dieting is not recommended during pregnancy. What you should do is optimize your nutrition by eating a well-balanced diet including lots of fruits and vegetables, and avoiding junk foods and sodas with many empty calories. This will allow you to gain healthfully and improve the condition of yourself and your fetus.
No. Women who start a pregnancy while overweight are not expected to gain as much as those who are underweight at the start, but we do not recommend "dieting" in the sense of calorie restriction. Eating "clean" by which I mean vegetables, whole grains, lean proteins and avoiding processed, sugary, or fatty foods will minimize weight gain without compromising nutrition.
Yes. The typical weight gain in pregnancy for a patient at her ideal wight is about 15-32 lbs. For the entire nine months. If a patient starts out overweight, her doctor may want her to gain no more that 15 lbs, this will depend on your ideal weight. Talk to your doctor. And remember, if it is safe, always exercise in pregnancy to help control excessive weight gain.
No. Pregnancy is not the time to try to lose weight. It is challenging enough to maintain a healthy diet and exercise while pregnant. Depending on your starting weight, your doctor will reccomend an appropriate weight gain; anywhere from 10 - 35 lbs.