It is possible that someone could attempt to do this, however if you can prove (which should be easy to do since all posts are stored on the blockchain) that someone posted the same content as you and tried to downvote you and posted it themselves and you posted a screencap and other members saw this it would have a negative impact for the person copying your post. If other influential people or a group of people see that a user took content without citing the original poster the karma will get back to them for sure.