Unexpected redefinitions

[epipping]

I have external-program installed via quicklisp. If I load it via ASDF, I get:

% sbcl --eval "(asdf:operate 'asdf:load-op :external-program)" --quit
This is SBCL 1.3.7.nixos, an implementation of ANSI Common Lisp.
More information about SBCL is available at <http://www.sbcl.org/>.

SBCL is free software, provided as is, with absolutely no warranty.
It is mostly in the public domain; some portions are provided under
BSD-style licenses.  See the CREDITS and COPYING files in the
distribution for more information.
WARNING:
   redefining RUN (#<SB-PCL:SYSTEM-CLASS COMMON-LISP:T>
                   #<SB-PCL:SYSTEM-CLASS COMMON-LISP:T>) in DEFMETHOD
WARNING:
   redefining START (#<SB-PCL:SYSTEM-CLASS COMMON-LISP:T>
                     #<SB-PCL:SYSTEM-CLASS COMMON-LISP:T>) in DEFMETHOD
WARNING:
   redefining PROCESS-INPUT-STREAM (#<SB-PCL:SYSTEM-CLASS COMMON-LISP:T>) in DEFMETHOD
WARNING:
   redefining PROCESS-OUTPUT-STREAM (#<SB-PCL:SYSTEM-CLASS COMMON-LISP:T>) in DEFMETHOD
WARNING:
   redefining PROCESS-ERROR-STREAM (#<SB-PCL:SYSTEM-CLASS COMMON-LISP:T>) in DEFMETHOD
WARNING:
   redefining PROCESS-STATUS (#<SB-PCL:SYSTEM-CLASS COMMON-LISP:T>) in DEFMETHOD
WARNING:
   redefining PROCESS-ID (#<SB-PCL:SYSTEM-CLASS COMMON-LISP:T>) in DEFMETHOD
WARNING:
   redefining PROCESS-P (#<SB-PCL:SYSTEM-CLASS COMMON-LISP:T>) in DEFMETHOD
% 

This does not happen for other packages (except for those that depend on external-program). Is something wrong with the build system perhaps?