Whence, according to its conventional definition, means from where, so the phrase from whence is logically redundant. But this doesn’t stop people from using from whence, a phrase that has been common for centuries. When hearing the sentence Whence came you?, one may feel something is missing—specifically, a preposition—even though the sentence is well constructed without it. That’s why from whence is so often used instead of whence alone, as in these examples:
Without warning, he dissolves into the blackness from whence he came. [Backstage]
The tide changed, returned to from whence it came, and is rising. [CounterPunch]
What made the horror of Kittel’s antisemitism is not just the words but from whence the words came. [Seattle Post-Intelligencer]
Of course, redundancy hasn’t stopped great writers from using from whence for centuries—for example:
From whence at pleasure thou mayst come and part. [Shakespeare]
From whence it follows, that where the publique and private interest are most closely united, there is the publique most advanced. [Thomas Hobbes]
Life lies behind us as the quarry from whence we get tiles and copestones for the masonry of to-day. [Ralph Waldo Emerson]
Other resources
“From whence” at World Wide Words
“The Whence Offense” at Columbia Journalism Review