This is normal with the ground truth flow 
Since the background in the image has no movement (or its movement is slighter) the part of the background that can be seen in the first image that got occluded by the moving object in the second image is the doubled area.
To make it simpler to understand, assume an image with a box in the middle with a still background (back ground with no movement). Assume that the box moves 5 pixels to the right from the first to the second image.
Now to warp the second image to the first image with the ground truth flow, you take a look at the coordinates of the first image, read the flow for each pixel, and copy the corresponding pixel’s color in the second and replace the color of that initial pixel in the first image.
In this case, since the back ground’s flow is zero, the part of the background that could be seen in the first image but not in the second image (i.e. got occluded with the object) is copied from the second image (showing parts of the moving object) to the that part of the first image. Also the whole box is copied to its initial position. So in the resulting image you will have the box + a part of the box that occluded those 5 pixels of the background in the second image.