"I know it is recommended to have some undergraduate coursework in Statistics. If I took AP statistics in high school and received college undergraduate credit for it (granted it was 10 years ago), does that count? Or do I need to retake a statistics class that I already technically have credit for?"
GSPP does not require prior quantitative training; however prior course work in introductory statistics, first-year calculus, and introductory economics is strongly recommended. We recommend these courses not just as an indicator of your current quantitative skills, but also as a gauge of your ability to handle the quantitative rigor once you're enrolled.
So, does an AP class "count"? It's not really a matter of counting, since there's no formal requirement. If you need to reach back to high school for evidence of your quantitative skills, though, it will be very difficult for the admissions committee to gauge your ability to handle core courses like Decision Analysis, Modeling and Quantitative Methods and The Economics of Public Policy Analysis.
Further, if there's no grade associated with a quantitative course on your transcript, only credits, it's of limited use to the admissions committee when evaluating your quantitative skills.
The more evidence you show of success in a quantitative environment (transcripts, GRE, letters of recommendation from an employer, etc.), and the more recent that evidence, the more competitive you will be as an applicant, and the more prepared you will be as a student.
Note: If you have already received your bachelor's degree, don't fret. We encourage and welcome applicants who complete quantitative coursework after they graduate at a University, local community college, or online, as long as it is through an academic institution and a transcript is issued.