Yes, that's it!
I accidentally added duplicate covers but I didn't know what I did. It was when I used Sigil to edit the text.
That's what I want to do again, but this time do it on purpose, dead tree cover and first edition cover for instance.
I just have to learn how to use Sigil then. I usually edit in a word processor and then convert to epub in Calibre, so I haven't had much use for Sigil so far.
I'll have a play with it now I know it will do what I want.
Thanks