Not sure if it is the rules but, logically, it should be their week 2 dance
a) because it would be unfair to ask the celebrities to recall their week 1 dances having recently learnt something else at this early stage,
b) there is no way the staging, costuming and make-up from Week 1 will be put in place for 180 minutes of dancing.
Then again, logic is not Strictly's strongest suit...