IIRC fast charging stops at 80% (unsure if this is always the case or there has to be a precedent like the phone overheating during charge) so, in theory, the double chime should still be heard up until 79% if it is indeed indicative of fast charging. But this is coming from someone who's never heard/noticed her iPhone chime twice. I guess I just don't pay attention! I think there's also a new feature in iOS 13 where charging stops at 80% if your phone gets too warm. I do notice my phone doesn't really charge past 80% in 30 minutes when keeping apps like Lyft up and running.