Checkout the content of the following thread:
Bug on the merit system?.
The content explains the issue, and the fix that was set in place:
Alright, you now can't merit the same post with the same amount within 60 seconds.
Refreshing the page after 60 seconds (in your case there is a 210 second gap) will cause you to send double the amount of sMerits (not double spend really, but rather double send).