This incorrect structure occurs sometimes when people have some kind a of vague sense that a sentence should not end with a preposition. They put the preposition someplace else in the sentence and then go ahead and put another preposition at the end of the sentence. Here are some examples:
It was a mystery of which he knew nothing about.
CORRECTED:
It was a mystery of which he knew nothing. (formal)
It was a mystery which he knew nothing about. (less formal)
He declared that there were secret rites in which he had never participated in.
CORRECTED:
He declared that there were secret rites in which he had never participated. (formal)
He declared that there were secret rites which he had never participated in. (less formal)
Another type of double preposition occurs with "off of." Both of these words are prepositions, so a phrase should have only one of them:
He fell off the log. (not off of)