Quite by accident, I noticed that when hovering over a 
on a post that I’ve already clicked, my cursor changes to a “general prohibition sign[1]” a.k.a. 
. A click made with this cursor to “unlike” a post results in no action taken.
EDIT: just tested by ing a new post and was able to immediatley un-. But I’m still unable to un- other posts.
Has their been an upgrade or other maintenance since you liked it?
That’s what I was wondering. Perhaps a database schema change made past likes immutable.
Or perhaps you’ve never been able to unlike something after a certain amount of time and I am just now noticing it.
That’s it - there’s a ten-minute window to undo a like, after which it becomes fixed (“post undo action window mins”). I don’t know which version this came in with, but I certainly saw it before the most recent upgrades.