The thank button, which has replaced the old like button (or whatever it was called), allows members to un-thank a thanked post, as the button reappears a couple seconds after being clicked. However, there is no immediate visual indication that the post was thanked; the only way to find if you already thanked the post is by the button's tooltip. If the button turned into a brown button with a down arrow icon, or gain a glow around the green button, or somehow have a different state for on and off, it would indicate that clicking it (again) would un-thank the post.
Improvements to the 'Thank' Button