This article prospects some avenues of reflection on the rare, but interesting, case where two or more states are committed to a substantively equal obligation under different treaty instruments. To what extent can it be said, in such a case, that these states are bound by that treaty-rule in their inter se relations? It is clear that if the rule at stake reflects general customary international law, all states are bound by it, and no problem arises. But what if the rule is merely conventional? The article first explores the rather complex concept of ‘equal obligation’. The measure of that equality must be fine-tuned in several ways, since a similar text does not convey necessarily the same substantive obligation. The article proceeds to give some solutions to the problem posed at the inception, exploring the tension between the rule on privity of treaty (pacta tertiis nec nocent nec prosunt) and the rule of non-formality in treaty-law, i.e., that an obligation can be assumed in several informal ways, by looking at the substance.