about as surprising as discovering that a mold used to cast a metal tool can also serve as a tool itself

Not at all suprising if either:

1. You have a complete neophyte designer who thinks that in addition to having a need for a tool cast, he wants to get at the same time a file to sharpen it and doesn’t care how it fits in the hand. The centuries of grief the abandonment of K.I.S.S. engineering principles will be directly attributable to such a unqualified designer. Or…
2. Things like the mold are developed piecewise by unintellegent agents from ancestor blobs and templates for blobs that serve a useful function are communicated to other agents for templates to continue the common descent with undirected changes accumulating over time. In such a model, if the blob has two functions, that makes it likely to outcompete a “designed” alternative which neglects one function for only modest gains for the other function.