I've had cameras in the past with very sensitive shutter buttons. Sometimes what was intended by me to be a half press was enough pressure to result in a full press. The resulting vibration of the camera as the mirror, shutter, and shutter reset mechanism were actuated was usually enough to disengage the full press and the effect was very much like what you describe. Practice a bit with your camera and see if that might be the case.

Regardless of the camera's configuration, if the mirror is cycling, the shutter is operating, and an image is being recorded to the memory card upon an actual half press of the shutter button, your camera is malfunctioning. There is no configuration option for the EOS 77D (or any other Canon EOS camera of which I'm aware) that enables an image to be recorded when the shutter button is half pressed. Images should only be recorded with a full press of the shutter button.

Like xenoid advises in the comments to the question, I'd recommend trying a wired cable release (you can get a generic for about $5 on amazon or eBay) with a two stage button. The cheaper wired releases would probably be better in this regard, as there is more of a difference in the "feel" between a half press and full press compared to your camera's shutter button.

• If the malfunction continues with a half press of the wired release, then the issue is not in your shutter button itself but in the way the camera responds to the shutter button's position.
• If the issue is not present when you half press the button on the wired remote, then the issue is probably in the contacts in the shutter button and the camera is interpreting that as a very short duration full press.