That actually is a bone of contention. I haven't seen this question, but there's another question, in which you have to find the oxidation state of Sulphur in H₂S₂O_3..

There one sulphur atom is doubly bonded to the another. Basically structure is same as H₂SO₄ with one S replacing one O. Now some books list the oxidation states as 0 and 4, while others do it as +6 and -2. The second by drawing analogy with sulphuric acid..

Let me check what JD Lee says, and get back to you..