(def asym-hobbit-body-parts [{:name "head" :size 3}
                             {:name "left-eye" :size 1}
                             {:name "left-ear" :size 1}
                             {:name "mouth" :size 1}
                             {:name "nose" :size 1}
                             {:name "neck" :size 2}
                             {:name "left-shoulder" :size 3}
                             {:name "left-upper-arm" :size 3}
                             {:name "chest" :size 10}
                             {:name "back" :size 10}
                             {:name "left-forearm" :size 3}
                             {:name "abdomen" :size 6}
                             {:name "left-kidney" :size 1}
                             {:name "left-hand" :size 2}
                             {:name "left-knee" :size 2}
                             {:name "left-thigh" :size 4}
                             {:name "left-lower-leg" :size 3}
                             {:name "left-achilles" :size 1}
                             {:name "left-foot" :size 2}])


(defn matching-part
  [part]
  (if (re-find #"^left-" (:name part))
  #{ {:name (clojure.string/replace (:name part) #"^left-" "right-")
      :size (:size part)}
     {:name (clojure.string/replace (:name part) #"^left-" "top-")
      :size (:size part)}
     {:name (clojure.string/replace (:name part) #"^left-" "bottom-")
      :size (:size part)}
    }))

(defn better-symmetrize-body-parts
  "Expects a seq of maps that have a :name and :size"
  [asym-body-parts]
  (reduce (fn [final-body-parts part]
            (into final-body-parts (set (into (into [] (matching-part part)) #{part}))))
          []
          asym-body-parts))

(pprint (better-symmetrize-body-parts asym-hobbit-body-parts))

;; Result:
;;[{:name "head", :size 3}
;; {:name "left-eye", :size 1}
;; {:name "right-eye", :size 1}
;; {:name "bottom-eye", :size 1}
;; {:name "top-eye", :size 1}
;; {:name "left-ear", :size 1}
;; {:name "right-ear", :size 1}
;; {:name "bottom-ear", :size 1}
;; {:name "top-ear", :size 1}
;; {:name "mouth", :size 1}
;; {:name "nose", :size 1}
;; {:name "neck", :size 2}
;; {:name "top-shoulder", :size 3}
;; {:name "left-shoulder", :size 3}
;; {:name "right-shoulder", :size 3}
;; {:name "bottom-shoulder", :size 3}
;; {:name "bottom-upper-arm", :size 3}
;; {:name "right-upper-arm", :size 3}
;; {:name "left-upper-arm", :size 3}
;; {:name "top-upper-arm", :size 3}
;; {:name "chest", :size 10}
;; {:name "back", :size 10}
;; {:name "top-forearm", :size 3}
;; {:name "bottom-forearm", :size 3}
;; {:name "left-forearm", :size 3}
;; {:name "right-forearm", :size 3}
;; {:name "abdomen", :size 6}
;; {:name "left-kidney", :size 1}
;; {:name "bottom-kidney", :size 1}
;; {:name "top-kidney", :size 1}
;; {:name "right-kidney", :size 1}
;; {:name "left-hand", :size 2}
;; {:name "top-hand", :size 2}
;; {:name "right-hand", :size 2}
;; {:name "bottom-hand", :size 2}
;; {:name "right-knee", :size 2}
;; {:name "top-knee", :size 2}
;; {:name "bottom-knee", :size 2}
;; {:name "left-knee", :size 2}
;; {:name "top-thigh", :size 4}
;; {:name "right-thigh", :size 4}
;; {:name "bottom-thigh", :size 4}
;; {:name "left-thigh", :size 4}
;; {:name "bottom-lower-leg", :size 3}
;; {:name "right-lower-leg", :size 3}
;; {:name "left-lower-leg", :size 3}
;; {:name "top-lower-leg", :size 3}
;; {:name "right-achilles", :size 1}
;; {:name "top-achilles", :size 1}
;; {:name "left-achilles", :size 1}
;; {:name "bottom-achilles", :size 1}
;; {:name "bottom-foot", :size 2}
;; {:name "top-foot", :size 2}
;; {:name "right-foot", :size 2}
;; {:name "left-foot", :size 2}]