As best I can determine, from memory, the double posts occurred while using the Tab key to move from the “Body of reply” to the “submit” button.
Even now, I’m finding that I am not consistently landing where I should.
In the cases where double posting occurred, I remained at the orignal “Post a reply” screen, but after submitting, discovered that my reply was already posted.
Stranger yet, in at least one instance, I noticed that it was posted to the wrong node! This may have happened more than once, but gone unnoticed because the node where the reply was 1st posted was outside the area being viewed by me.