How normal would it be for one of my breasts to be twice the size of the other?

While breast sizes.. Do vary, if one breast is literally twice the size, there are some things that a plastic surgeon can help you with to improve your symmetry, such as reducing the larger side or enhancing the smaller side. Best to have an in-person consultation, though.
Lift/implants/reduce. This comes down to an analysis of the differences: it may be simply making the larger breast smaller or the smaller breast larger. However it more likely involves a combination of both as well as analyzing the need for a lift or repositioning of the breast creases or nipples or parenchyma. Perfect symmetry is not a realistic goal and will set you up for disappointment. Goal: less asymmetry:).