Your brain simply will not allow you to actually see both images at the same time.
It will bounce back and forth between images but never allowing both images to be viewed at the same time.
This is called the figure reversal process.
Psychology and Mental Health people suggest that even if we do not notice duck and rabbit, our brain may have actually registered the unreliability of the image subconsciously, and decided, in effect, not to spread the news.
In this view, your brain itself is in on the trick.
The only dupe left in the room is you.