There is no reference anywhere in the decision on Durban Platform to symmetry of obligations by the Parties, even if the post 2020 arrangements have to apply to all Parties. This may have been the intention of some of the parties. But, the formulation as agreed does not disclose that a single outcome will be based on symmetry of obligations or commitments.
Since the post 2020 arrangements have to be evolved under the Convention, they will need to respect the principles of the Convention and there will be space for differential obligations. Even the Kyoto Protocol which is an existing legally binding agreement recognizes the difference in obligations.