Insert metadata as page at start of book adds does not replace (mobi to mobi)

I wanted to put a new description at the start of one of my books. I changed the metadata then I went and reconverted (mobi to mobi) with Insert metadata as page at start of book I then went and checked the finish product and now I have two descriptions. Insert metadata as page at start of book.

Can someone tell me what I did wrong. I am using calibre 0.8.60