Can a CAFCI be double-lugged?
Nope!

Siemens QP breakers, with basically no exceptions, cannot be double-lugged the way you see there.  Fortunately, this is easy to fix.  All you will need is a couple of 12AWG stranded THHN pigtails (one black, one white) and a pair of wirenuts suitable for 3 12AWG wires each, as well as an inch-pound torque screwdriver to set the torque on the breaker lugs properly.  Turn the breaker in question off, unfasten the wires from the lugs, nut each pair (black and black, white and white) together with its corresponding pigtail color, insert the pigtails into the correct lugs (black in the top lug, white in the bottom lug), and torque the lugs back down to the labeled torque values using your torque screwdriver.