Spotted yesterday on yahoo.com:
What’s the issue? It’s the use of off of, which some think is wrong and others say merely lacks concision.
The American Heritage Dictionary says:
The compound preposition off of is generally regarded as informal and is best avoided in formal speech and writing: He stepped off (not off of) the platform.
While Merriam Webster states:
The of is often criticized as superfluous, a comment that is irrelevant because off of is an idiom. It is much more common in speech than in edited writing and is more common in American English than in British.
Aha! There’s the reason for the superfluous of! I’m reading yahoo.com, which isn’t exactly “edited writing.”