[MLv2] Ensure `:name` is unique on returned columns in the FE (#39118)

This is FE-only because this is a compatibility issue with the (legacy)
metadata coming back from the QP.

In time, the FE logic should be ported to MLv2 or made to depend on
things other than the `:name` field. MLv2 code itself (including BE
usage) should be based on the (already unique)
`:lib/desired-column-alias`.

Fixes #37517.

src/metabase/lib/js.cljs

@@ -647,9 +647,15 @@
 (defn- returned-columns*
   "Inner implementation for [[returned-columns]], which wraps this with caching."
   [a-query stage-number]
-  (let [stage (lib.util/query-stage a-query stage-number)]
+  (let [stage          (lib.util/query-stage a-query stage-number)
+        unique-name-fn (lib.util/unique-name-generator)]
     (->> (lib.metadata.calculation/returned-columns a-query stage-number stage)
-         (map #(assoc % :selected? true))
+         (map #(-> %
+                   (assoc :selected? true)
+                   ;; Unique names are required by the FE for compatibility.
+                   ;; This applies only for JS; Clojure usage should prefer `:lib/desired-column-alias` to `:name`, and
+                   ;; that's already unique by construction.
+                   (update :name unique-name-fn)))
          to-array)))
 
 (defn ^:export returned-columns

test/metabase/lib/js_test.cljs

@@ -517,3 +517,17 @@
         (testing "unless it's missing in the input"
           (let [di (lib.js/display-info query -1 (dissoc discount :fingerprint))]
             (is (not (gobject/containsKey di "fingerprint")))))))))
+
+(deftest ^:parallel returned-columns-unique-names-test
+  (testing "returned-columns should ensure the :name fields are unique (#37517)"
+    (let [query (-> (lib/query meta/metadata-provider (meta/table-metadata :orders))
+                    (lib/join (lib/join-clause (meta/table-metadata :orders)
+                                               [(lib/= (meta/field-metadata :orders :id)
+                                                       (lib/with-join-alias (meta/field-metadata :orders :id)
+                                                         "Orders"))])))]
+      (is (= #{1}
+             (->> (lib.js/returned-columns query -1)
+                  (map :name)
+                  frequencies
+                  vals
+                  set))))))