Please site admin, don't save a duplicate message sent to someone. Sometimes when I reply, it shows that the message got sent twice, even after the Send button starts spinning and isn't tappable. I've seen duplicate messages from others too.
There's no way it was a double-tap, and even if the message got sent twice, it shouldn't save twice. And if it does, then it shouldn't show twice, and the duplicate can be deleted.
I work on websites, I know this can be prevented, fixed, or cleaned up. It looks really bad and is completely avoidable.
I agree this sometimes happens. In the meantime, it is best to tell the individual something like "Sorry the site automatically double sent."
This should probably be posted to the administrators.