[css-values] repeated values with the double-bar (||) combinator are invalid?

In section 2.2 of the CSS Values spec, the double bar combinator is defined
as [1]:

  "A double bar (||) separates two or more options: one or more
   of them must occur, in any order."

So a || b || c implies that "a b", "b c a", and "b" are all valid.
However, I'm not sure whether repeated values are allowed or invalid
(e.g. "a a"). Seems like it should be invalid but I can't find any wording
that
clearly disallows this.

Cheers,

John Daggett

[1] http://www.w3.org/TR/css3-values/#component-combinators