"I know about you."
Translation:Je connais des choses sur toi.
"I know you" or "Je te connais" signifies that you and I have met before (but perhaps didn't realize it at first). "I know ABOUT you" or "Je sais des choses sur toi" (or "J'en sais de toi", "On m'a dit des choses sur toi", etc.) signifies that you and I have NOT met before, but that I have gained information about you some other way, for example through gossip, overheard conversations, a file, or a newspaper report.
"I know about you" suggests that I know something that you would like to keep secret, or that I have heard you have a bad reputation. It might be said as a flirting remark. Duolingo offers as another possible translation: "J'en sais de toi." Does that have the same flirtatious connotation?