When using the function add_custom_ext the callbacks are stored using indices that depend solely on their function signature, as implemented on cached_ex_index. More specifically, a TypeId of the signature is used to generate the key.

This means that two different callbacks with the same signature will overwrite each other.
The key for the index should depend on its identifier (a.k.a. "extension type") as there should be a one-to-one relation between an extension type and its callbacks.