Welcome to Android Central! Which phone? Does it have the option for app cloning (i.e., creating two separate instances of an app so that you can have two different accounts logged in)?
Another possibility is that you inadvertently created a Progressive Web App from the Facebook website. Go to Settings>Apps, and see if Facebook is listed twice there. If it is, and if one of them is really small in size, then that smaller one is the PWA, and can be deleted.