If you have a previous conviction for DUI--or even an arrest with no conviction--the authorities will look at your case much differently than they will a first-offense. This is because they are human, and do not want blood on their hands. If they go too easy on someone with a proven record of drinking and driving, they risk being responsible for a human life, if you get into an accident.
Do not underestimate the sense of responsibility that most prosecutors and judges feel when they are dealing with someone who has had a DUI before. There are definite things that need to be done to mitigate that prejudice against you. And we know what they are.