Multiple posts at once, even if the user shouldn't ignore the flood interval

As phpBB board administator, I notice users sometimes post the same message twice during the same second, or within few seconds.

Actually, when you click on the "submit" button several times at the post editor, the same text can be published twice or more. The issue occurs as well with new topics as replies.

Setting a flood interval and prevent the users to ignore it doesn't change anything. Thus, it's definitely a bug.

The phenomenon is not new, and it's still present on phpBB 3.3.9