Since the recent changes to this site, when you respond to a post, the box you type into does not get "wiped" when that posts successfully.
Therefore it is easy to think it has not posted and re-post.
This appears to be why we have had at least dozens of duplicate responses here.
So please please please have a look at the whole thread and check whether your response has actually posted, maybe by refreshing the page?, before trying again.
Otherwise it simply causes confusion and is another job for your friendly neighbourhood admins to deal with.
Image: Beth Ditto at the Cannes Film Festival 2010 Georges Biard