Also, I have noticed another duplication (not sure if this is related to the above) on the post page. At the end of every post, it appears twice the “next and previous post”.
the Category Archive titles are displayed using a Block Element found in Appearance > Elements ( or you can see the ones active on a page from the Admin Bar > Elements menu ).
the one you need is titled: Archive Header Block
Can you edit that and make sure the content has not been duplicated ?