You already know “Boom Clap,” the lead single from the soundtrack for summer blockbuster The Fault in Our Stars (which Charli called a “full-on, amazing cry fest”). It’s by far the biggest solo release of Charli’s career: her second song to reach the top spot on the Billboard Hot 100 and her first song on the charts as the lead artist. What you may not know is that Charli originally wrote it (along with Patrik Berger) for placement on Hilary Duff’s forthcoming comeback album, and only made it her own song after it was rejected.

“My number one project as a songwriter is to write for Hilary Duff,” Charli told MTV UK this summer. “We sent over ‘Boom Clap’ but they came back and were like, ‘This isn’t cool enough for Hilary.’ I was like, crushed…. What a life-ruiner.” Turns out that Duff missed the memo, and says she would’ve taken the song if she had known about it. “I didn’t know that I had an option to have it,” Duff told Popjustice. “It’s my favorite song on the radio right now. If she wants to give it to me now, I’ll still take it!” Her loss is Charli’s gain. The song has over a million digital sales and will reappear on her third album, Sucker (out Dec. 16).