Honestly, I did experience like that but not in bitcointalk when you send merit or involving bitcointalk.org. Base on what I experience it is a bit similar to what you had experienced. You did send merit and sent successfully right? but after the merit sent, you refresh the page and then it send merit to the same person.
My explanation below might be happening/the issue in your experience.
Click
TASK=Going to merit USER A
You are directed to the page where your incoming and outgoing Merits are displayed,
Click
TASK= SENDING MERIT TO USER A
submission is successful
(SAVED) LAST TASK= SENDING MERIT TO USER A
submission is successful
REFRESHING THE PAGE
It will repeat the last task you made which the last task you did was sending merit to user A.
TASK= SENDING MERIT TO USER A
submission is successful
Unless after you submit but you did not refresh the page then double submission won't happen.
There is a possibility that double submission did happen when you refresh the page. Sometimes while sending and then you refresh, it will resubmit what you did in the first place.
Edited: Your issue with the double merit submission is the same as this one -
https://www.quora.com/How-do-you-prevent-a-form-from-resubmitting-when-the-page-is-refreshed