Canon EOS 5Ds Instruction Manual page 477

Multi Shot Noise Reduction cannot be set.
If the image-recording quality is set to RAW or RAW+JPEG, [Multi
Shot Noise Reduction] cannot be set.
The Live View image or movie shooting image is
not displayed during multiple-exposure shooting.
If [On:ContShtng] is set, Live View display, image review after image
capture, or image playback is not possible during shooting (p.226).
Live View shooting ends when multiple exposure
shooting is performed.
If you perform Live View shooting with [On:ContShtng], Live View
function will end automatically when the first exposure is taken. Take
the second and subsequent exposures while looking through the
viewfinder.
The multiple-exposure image is shot in 1 quality.
When the image-recording quality is set to 41 or 61, the
multiple-exposure image will be recorded in 1 quality (p.233).
When I use the <f> mode with flash,
the shutter speed becomes slow.
If you shoot at night when the background is dark, the shutter speed
automatically becomes slow (slow-sync shooting) so that both the
subject and background are properly exposed. To prevent a slow
shutter speed, under [z1: External Speedlite control], set [Flash
sync. speed in Av mode] to [1/200-1/60sec. auto] or [1/200 sec.
(fixed)] (p.248).
Troubleshooting Guide
477