You shouldn't believe contradictions. For any given proposition, you should disbelieve either it or its negation. This claim is ambiguous. It might mean the wide-scope disjunctive requirement:
(WD) You ought to disbelieve either one of P or not-P (and it doesn't matter which, at least for the purposes of this requirement).
Or it might instead be proposing a disjunction of narrow scope requirements:
(ND) It is either the case that you ought to disbelieve P, or that you ought to disbelieve not-P.
For ND, it does matter which option you choose. One of them is the correct option, we just don't know which! WD, by contrast, can be satisfied equally well by either option. (Footnote 27 of Kolodny's 2005 illustrates this difference nicely. I can quote it if anyone wants further clarification.)
I find wide-scope requirements like WD interesting because, if normative, they open up a new aspect of (non-evidential) epistemic assessment. They might lead us to assess beliefs against internal standards of coherence and consistency, rather than the external standard of truth. I used this idea in an earlier essay to argue that there could be non-evidential reasons for belief: after all, even if there are no reasons at all to think that P is false, the mere fact that you believe not-P would seem to give you reason to disbelieve P, since doing so would bring you to satisfy requirement WD.
But I don't know how plausible that is. If all the evidence supports P, we might think it more plausible to simply insist that you ought to believe P, and - further - that there's nothing at all to be said in favour of disbelief, no matter that it contradicts your prior (ill-founded) belief that not-P. If you're more sympathetic to these claims, then you'll likely prefer ND to WD as the proper form of our non-contradiction rule.
Incidentally, this should not be confused with the simple narrow-scope conditional requirement:
(NC) If you believe that P, then you are rationally required to disbelieve not-P.
NC is plainly false as a universal principle. Sometimes rationality requires us to reject our prior beliefs in favour of their negations. This lends support to the wide-scope reading of the conditional:
(WC) Rationality requires that: if you believe that P, then you disbelieve not-P.
WC, unlike NC, may be satisfied by rejecting the antecedent (belief that P) as an alternative to fulfilling the consequent (disbelieving not-P). In fact, WC is logically equivalent to WD above (well, if we treat the earlier 'ought' as merely meaning "rationally required", leaving open the question of whether these requirements are genuinely normative). But this reminds us that there is a second way to revise NC. Rather than converting it into a wide-scope requirement, we might instead replace it with a disjunction of narrow-scope requirements, as in ND. So the problems with simple "NC"-style narrow-scope requirements need not lead us to accept wide-scope requirements.