Yeah it happened to me several times. Luckily I usually give only 1 merit so even when it happens, its not a big deal and it doesn't usually empty my smerit.
When you refresh or click Send, within 60 seconds, it will show the message.
" You just merited that post! Did you double-click? ".
Is it something relatively new? I am asking because I could swear that I didn't see any message each time it happened (last time it happened was probably more than a year ago).