I also liked your observation two times. One way to replicate this is to open several pages in the browser of an observation you have not faved and then you can fav it in all pages because the button is active in all of the pages. Obviously the site does not check whether it has already been faved. Happens to annotations, too:
While these I did on purpose using several opened pages, it happened to me inadvertently (some time ago) on a single page with annotations. They are sometimes slow to register - there’s a progress icon:
But sometimes after the icon disappears, the drop down menu reappears as if the annotation has not registered. Sometimes it has not registered indeed, but other times - it has registered and now you can choose it again, and sometimes even again. And when you reload the page you will see the same annotation several times.