;; It was a package problem.  Those "duplicate" slot names were from different packages.  The solution was to modify uri-to-fluid-object to specify a symbols package

(defun uri-to-fluid-object (uri &key (method :get) parameters)
  (multiple-value-bind (body status headers)
      (drakma:http-request uri :method method :parameters parameters)
    (unless (= 200 status)
      (api-error "The request for ~a failed with status code ~a." uri status))
    (json:with-decoder-simple-clos-semantics
      (let ((json:*json-symbols-package* 'weather))	;; This is what changed!
	(values (json:decode-json-from-string (babel:octets-to-string body))
		status
		headers)))))