"Off of" is not uncommon in other phrases even when the "of" is not needed, e.g., "I washed the mud off of my car" or "I took that appointment off of my schedule."
Mixing parts of unrelated or semi-related phrases is not unusual. My favorite is "cast in stone."
Not as much as the misuse of the apostrophe - especially in the UK.
Back in the late 1990s I heard it used in Cambridge Mass and it appeared that "based off of" meant "derived from" to some people working there. A rather dim bunch, I thought, since as well as using such bizarre phrases they thought that the way to win an argument was to scream and swear and generally behave disgracefully in the hope that they could be more disgraceful than their opposition. That, in my experience, was not how competent engineers and scientists debate, so I found myself strongly inclined to regard their vocabulary, including the phrase Julie asked about, as decidedly horrible. I've never heard that phrase anywhere else - not in Menlo Park, or San Francisco, or Los Angeles, (or Seattle, or Chicago, or New Hampshire, or New York) so maybe Eugenia's suggested attribution of it to silicon valley is off target or maybe I haven't spent enough time in the USA (haven't been there since 2007) to know where the expression is used.
Thanks for the feedback. Seems I'm in good company.
Yes, but not as much as "all intensive purposes." :-D
Have seen pretty often and edit just as often.
Also very popular: Someone is "based out of" a particular geographical area, as opposed to "based in." So very strange.
I would suggest the change to ‘based on’ as well. It’s a reminder for us writers and editors that people typically write the way they speak!
Fear not. The Corpus of Contemporary American English has just 41 records for ‘based off of’. It has 79,933 for ‘based on’.
I hear younger people using it all the time. I still change it in written text, but I suspect it's the wave of the future.

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