IIRC double or indeed any number of consecutive semi-colons in CSS files are not errors in CSS level 2.1 and 3
And entering the following CSS in the W3C validation service confirms my memory
Code:
body {
color: red;;
background: green
}
And inserting a double semi-colon in the very first rule in a stylesheet and opening the book with a suitably ancient ADE 1.7 shows no problems.
So I have to conclude this is just epubcheck being useless as usual.