diff --git a/backend/src/app/storage.clj b/backend/src/app/storage.clj
index 218835149..10e289b7e 100644
--- a/backend/src/app/storage.clj
+++ b/backend/src/app/storage.clj
@@ -274,7 +274,7 @@
             (let [min-age (db/interval min-age)
                   rows    (db/exec! conn [sql:retrieve-deleted-objects-chunk min-age cursor])]
               [(some-> rows peek :created-at)
-               (some->> (seq rows) (d/group-by' #(-> % :backend keyword) :id) seq)]))
+               (some->> (seq rows) (d/group-by #(-> % :backend keyword) :id #{}) seq)]))
 
           (retrieve-deleted-objects [conn]
             (->> (d/iteration (fn [cursor]
@@ -383,7 +383,7 @@
                             (mapv #(d/update-when % :metadata db/decode-transit-pgobject)))]
               (when (seq rows)
                 [(-> rows peek :created-at)
-                 (d/group-by' get-bucket :id rows)])))
+                 (d/group-by get-bucket :id #{} rows)])))
 
           (retrieve-touched [conn]
             (->> (d/iteration (fn [cursor]
diff --git a/common/src/app/common/data.cljc b/common/src/app/common/data.cljc
index eb74bd3a4..08c952d93 100644
--- a/common/src/app/common/data.cljc
+++ b/common/src/app/common/data.cljc
@@ -597,19 +597,10 @@
 
 
 (defn group-by
-  ([kf coll] (group-by kf identity coll))
-  ([kf vf coll]
-   (let [conj (fnil conj [])]
-     (reduce (fn [result item]
-               (update result (kf item) conj (vf item)))
-             {}
-             coll))))
-
-(defn group-by'
-  "A variant of group-by that uses a set for collecting results."
-  ([kf coll] (group-by kf identity coll))
-  ([kf vf coll]
-   (let [conj (fnil conj #{})]
+  ([kf coll] (group-by kf identity [] coll))
+  ([kf vf coll] (group-by kf vf [] coll))
+  ([kf vf iv coll]
+   (let [conj (fnil conj iv)]
      (reduce (fn [result item]
                (update result (kf item) conj (vf item)))
              {}