  Double-soft theorems, like its single-soft counterparts, arises from the
underlying symmetry principles that constrain the interactions of massless
particles. While single soft theorems can be derived in a non-perturbative
fashion by employing current algebras, recent attempts of extending such an
approach to known double soft theorems has been met with difficulties. In this
work, we have traced the difficulty to two inequivalent expansion schemes,
depending on whether the soft limit is taken asymmetrically or symmetrically,
which we denote as type A and B respectively. We show that soft-behaviour for
type A scheme can simply be derived from single soft theorems, and are thus
non-preturbatively protected. For type B, the information of the four-point
vertex is required to determine the corresponding soft theorems, and thus are
in general not protected. This argument can be readily extended to general
multi-soft theorems. We also ask whether unitarity can be emergent from
locality together with the two kinds of soft theorems, which has not been fully
investigated before.
