I think it's fairly common. I notice my BF does it a bit, though he borrows from sources so obscure it's hard to tell sometimes.
I'm currently reading a book on AS and long term relationships, and this stood out to me:
"You mean you don't see any future for us?" I immediately went on red alert with that high-pitched tone of voice that he doesn't seem to hear. He is always this condensed with his speech, but his answer alarmed me. He responded, "Of course we'll be together forever, but I don't expect anything in particular." Because of my persistently quizzical look (and because I wasn't going away) he responded with, "Things will continually improve. Our fights will dwindle and our love will grow as we learn to live with each other. Our marriage is sacred and I'm still thrilled to have you by my side." To the passerby, it might sound like he had a vision of marriage, but I could tell from his phrasing that he was repeating words he'd heard in the past. Silence. I didn't know whether to call his bluff on parroting words he'd learned elsewhere or just accept it. This is a common dilemma for me. I don't want to squelch his attempts at communicating, but I also don't want to accept others' words as his own. Again, he saw that I wasn't going away and he turned towards me. Direct eye contact - painful for him, but a generous gesture toward me. He clarified with something I know was uniquely his idea, an honest insight into his views. He said, "You've become a part of me," and the conversation was over."
(Ashley Stanford. Asperger Syndrome and Long-term Relationships )
So, I think 'parroting' might actually be a common thing, but I don't know enough about it to say with any sort of authority.