Sometimes you can optionally leave out prepositions in a phrasal preposition pair. The of in off of is one of the most common.
If you take the word out of the sentence and see if it still makes sense. If so, it's extraneous. We won't go so far as to say that the "of" in "off of" is completely unnecessary (and semantically void); we're just saying that this is an example of a reduced form.
Kind of like how you can leave out "that" in relative clauses: "a boy I like" vs. "a boy that I like."
"My neck and back are pretty mad at me for jumping off of that 50-foot cliff."
"My neck and back are pretty mad at me for jumping off that 50-foot cliff."
Both of these sentences are grammatically correct, but there's some debate about whether "of" should always be excluded. Pay attention to this. We certainly don't mind either way, but your English teacher might.