Yes I did have blanks. I replaced a blank field in one of the categories with a zero and that has resolved it showing duplicate images. I've since replaced the blanks with URL to the images I want to show. All seems to be working well.
In the admin panel - Edit Categories, the comment for Category Avatar File Path says:
(leave blank to use default program option otherwise place a full url to an image to use as an avatar. It is best to have these set to all the same size.)
So should this read set to zero and not leave blank?