I used to double post a while ago and I forget what it was that caused that. It was not my browser settings though. I think maybe I was hitting the button because I didn't think it did post in the first place.
Might be a slow connection and the response from Stamp Community doesn't come back in a timely manner so our brains think it didn't work.

Could be that these are not Star Fleet computers either and go so slow that First Officer Spock, used to the speed and efficiency of the usual input devices, his brain moving along faster than a humans, or in different patterns of thought, considers the said computers defective or deficient in some subconscious way.

Could be a Borg plot.

Could be a new life form.

Could be all the stuffing is up one end.

Some things in life are not meant to be explained. They just are.
Could be spock is going where no man has gone before . . .