By default most system-provided definers such as cl:defun, cl:defmacro, cl:defmethod and so on signal a warning when they redefine an existing definition. You can bind or set *redefinition-action* to eliminate such warnings or make it signal error instead.
Also, the system is configured to protect symbols in implementation packages against definition and redefinition as described in 7.7.2.2 Protecting packages.
LispWorks® User Guide and Reference Manual - 01 Dec 2021 19:30:20