Does an iPhone Photo Stream upload store a duplicate of the photo locally? That is to say, does a photo uploaded to Photo Stream and/or a Shared Photo Space take up twice or thrice the space on my phone, or does each instance point to the same single data store?
Yes one copy is stored for each "service" (for lack of a better word). So if it exists in your Camera Roll, Photo Stream, and a shared Photo Stream, there are three physical files on your phone.