[time.duration.nonmember]/1 states

In the function descriptions that follow, CD represents the return type of the function.

From what I could find, many definitions of CD in the paragraphs of [time.duration.nonmember] were lifted to [time.duration.nonmember]/1 as cited above. That works for all other paragraphs, but not for [time.duration.nonmember]/10, which the change rendered ill-formed:

template<class Rep1, class Period1, class Rep2, class Period2>
  constexpr common_type_t<Rep1, Rep2>
    operator/(const duration<Rep1, Period1>& lhs, const duration<Rep2, Period2>& rhs);

Returns: CD(lhs).count() / CD(rhs).count().

In this case, we want CD to mean common_type_t<duration<Rep1, Period1>, duration<Rep2, Period2>>. That way, the division has the expected semantics of dividing two quantities of the same dimension.