Fans and friends of Duff's were quick to comment, including her sister, Haylie, who wrote, "Ummm, I thought this was something else."
"Hahah! Me too," added Duff's Younger co-star, Debi Mazar.
ET has reached out to Duff's rep for comment.
Engaged or not, it's certainly been an exciting year for the former Disney star, who announced in June that she and Koma were expecting a baby girl. She also shares 6-year-old son Luca with her ex-husband, Mike Comrie.
Hear more on Duff's second pregnancy in the video below.