I mostly agree with DGL. 안 하면 안 돼 is an indirect way of saying "해야 돼". This one has just striked me. "엄마 나갔다 올 테니까 동생이랑 싸우면 안 돼. 공부 안 해도 안 돼."

double negatives.. sorry but we don't say '안해도 안돼'. instead we use '안하면 안돼'. yes they are equivalent. 안하면 안돼 = 해야돼. I may be wrong, but I cannot think of a case when double negatives mean (strong) negative in Korean. double negatives almost always (if not always) mean positive.