Hi, Hannah. My apologies for taking a whole year to answer your question. I just caught on to this Q&A portion of the page.
Yes, all my books are stand alone thrillers. The only time I've used a character twice was "Sally Justice" the always-angry CNN law/crime analyst who showed up in TERRIFIED and again in YOU'LL MISS ME WHEN I'M GONE. Thanks for your question, Hannah!