Avicenna, Philosophia Prima, Tractatus IX, Capitulum VII, p. 512, line 15-18:
Tu autem scis quod, cum intendis in aliquid quod nimis est tibi cordi, si praesentatur tibi aliquid in quo est voluptas et fecerint te eligere unum de duobus, quod contemnes voluptatem, si fueris nobilis animi
I can't understand the function and the meaning of quod. It seems to me that without this quod, the sentence is still meaningful.
Sebastian is right. It's repeated in order to resume the sentence after a lengthy clause. We see this even in Classical Latin. I hesitate to call it a mistake, since it is so often deliberately chosen in order to alert the reader that the sentence has resumed.
From Wills, Repetition in Latin Poetry, p. 66:
Cicero also uses gemination to resume unfinished sentences after a digression or intervening clause.
Note that he's using 'gemination' to mean 'the repetition of a word (or words) in identical form in the same unit (clause or sentence).' Moreover, and this directly addresses your concern above, gemination occurs when 'the repeated element can be left out without any syntactic loss.' (p.11)
He cites Roschatt (Ueber den Gebrauch) 220-229 and Wernicke (De Geminationis) 25-30, but I don't have immediate access to those books (looks like they weren't scanned by Google Books yet).
