According to some sources according to IUPAC a double bond has a higher priority than a single bond. However, some others state that a double bond can be interpreted as two single bonds leading to two of the elements further in chain, unattached to any other. Thus, depending on which principle is used the R/S configuration varies, for example, 2-chloro-3,3-dihydroxypropanal (attached).
So which is the correct one and why is?
Unfortunately I do not reckon the sources, yet Wikipedia proposes the first method and second comes from some sort of Russian org chem book.