Who care about one merit send to one post? You must be more careful if you have intention to send someone 10, 20, or 50 merits, when post quality will be a serious issue. For the post you mismerited, just let it be.
but in some case, some people make use of this to do transaction sell merit for money.
We know that there have been many cases that have sprung up so there is truth in what OP says.
even if it's just a mistake, you shouldn't repeat it.
but i though it wont be easily to do, because need 2 time confirmation for giving Smerit