Lyndsey
You can join up to 3 challenges at the same time. I had this happen to me before too before with the “already uploaded” message. It was explained to me by customer support that a new photo can only be taken after 24 hours have passed from the previous photo you took in that challenge, assuming that this wasn’t the first time you’ve uploaded a photo in the 10k challenge. So, if you took a photo at 8am one day, you’d have to wait until after 8am the next day to take the next photo. Or, if you took a photo at 8pm one day, you’d have to wait until after 8pm the next day to take a photo. If you have issues uploaded after waiting, I’d recommend contacting customer support for assistance.