Anarchism is the process of identifying hierarchical structures, questioning their legitimacy, and if it cannot justify itself, abandoning it and replacing it.
Patriarchy is a form of hierarchy that cannot justify itself, and therefore it should be abolished. This fits exactly with the normal anarchist definition. Any anarchist should therefore be a feminist (egalitarian; anti-patriarchy), and thus anarcha-feminism is a double term.
Sadly though, the fact that anarcha-feminism exists means some people who call themselves anarchist make an exception for patriarchy. This is what is called brocialism.