Double flash is commonly used in film projectors so that each of the 24 f/s is shown twice; a total of 48 f/s. This means the movie presentation has less flicker. Triple flash is better still with a frame rate of 72 f/s.

When presenting 3D cinema, the left and right eyes want motion and the parallax to appear at the same time but the sequential frame presentation of 3D, often using a single projector, naturally offsets motion timing. Double, or better triple, flash improves the motion portrayal. Here total frame rates are double that of 2D, so:
single flash is 48 f/s
L1, R1, L2, R2, etc.

double flash is 96 f/s
L1, R1, L1, R1, L2, R2, L2, R2 etc

triple flash is 144 f/s
L1, R1, L1, R1, L1, R1, L2, R2, L2, R2, L2, R2 etc.

Note that the cine player offers only 24 left and right frames/s. It is the job of the projector to present each frame two or three times. Of course, the projector has to be capable of clearly showing frames at that rate.