I just did.
OK, now it should be impossible no matter how much you try.
Yes, now working fine. I just tried to merit you with different 2 window but it's not possible now. I think now merit abuser can't excuse that I have sent by mistake. I have remembered few abuser excuse like this.
Alright, you now can't merit the same post with the same amount within 60 seconds.
Different amounts still work, I'm fine with this workaround to protect users against their double click habits 
Doesn't matter if different amount, because amount different means you are fully aware that you are going to send another amount of merit to same post.