Double clicking or reloading the page both had the same result. It's actually the reason theymos implemented the 60 second delay, but that doesn't stop all cases as you found out.
Not really. If you see the OP's image, there is a difference of less than a minute but between two different posts. The merit that I sent to the same post when refreshing the page is about 7 minutes apart from the other.
I just tried to do the same and couldn't, even waiting for longer than 1 minute (I gave you two merits but because I tried twice separately).
I would lock the thread but this is Meta, I don't think it will last long here without being locked if there isn't some meaningful discussion going on.