diff --git a/server/src-lib/Data/HashMap/Strict/Extended.hs b/server/src-lib/Data/HashMap/Strict/Extended.hs
index eaff0dfba..9902cadd0 100644
--- a/src-lib/Data/HashMap/Strict/Extended.hs
+++ b/src-lib/Data/HashMap/Strict/Extended.hs
@@ -7,7 +7,6 @@ module Data.HashMap.Strict.Extended
   , groupOnNE
   , differenceOn
   , lpadZip
-  , mapKeys
   , unionsWith
   ) where
 
@@ -54,20 +53,6 @@ lpadZip left = catMaybes . flip A.alignWith left \case
   That    b -> Just (Nothing, b)
   These a b -> Just (Just a,  b)
 
--- | @'mapKeys' f s@ is the map obtained by applying @f@ to each key of @s@.
---
--- The size of the result may be smaller if @f@ maps two or more distinct
--- keys to the same new key.  In this case the value at the greatest of the
--- original keys is retained.
---
--- > mapKeys (+ 1) (fromList [(5,"a"), (3,"b")])                        == fromList [(4, "b"), (6, "a")]
--- > mapKeys (\ _ -> 1) (fromList [(1,"b"), (2,"a"), (3,"d"), (4,"c")]) == singleton 1 "c"
--- > mapKeys (\ _ -> 3) (fromList [(1,"b"), (2,"a"), (3,"d"), (4,"c")]) == singleton 3 "c"
---
--- copied from https://hackage.haskell.org/package/containers-0.6.4.1/docs/src/Data.Map.Internal.html#mapKeys
-mapKeys :: (Ord k2, Hashable k2) => (k1 -> k2) -> HashMap k1 a -> HashMap k2 a
-mapKeys f = fromList . foldrWithKey (\k x xs -> (f k, x) : xs) []
-
 -- | The union of a list of maps, with a combining operation:
 --   (@'unionsWith' f == 'Prelude.foldl' ('unionWith' f) 'empty'@).
 --