diff --git a/lib/elixir/lib/keyword.ex b/lib/elixir/lib/keyword.ex
index f6f57e8b7c..d6c8c24128 100644
--- a/lib/elixir/lib/keyword.ex
+++ b/lib/elixir/lib/keyword.ex
@@ -193,6 +193,15 @@ defmodule Keyword do
       iex> Keyword.new([{:a, 1}, {:a, 2}, {:a, 3}])
       [a: 3]
 
+      iex> Keyword.new([{:a, 1}, {:b, 2}, {:a, 3}])
+      [b: 2, a: 3]
+
+      iex> Keyword.new([{:a, 1}, {:b, 2}, {:a, 3}, {:c, 4}, {:b, 5}, {:a, 6}])
+      [c: 4, b: 5, a: 6]
+
+      iex> Keyword.new([])
+      []
+
   """
   @spec new(Enumerable.t()) :: t
   def new(pairs) do
@@ -214,12 +223,17 @@ defmodule Keyword do
   """
   @spec new(Enumerable.t(), (term -> {key, value})) :: t
   def new(pairs, transform) when is_function(transform, 1) do
-    fun = fn el, acc ->
-      {k, v} = transform.(el)
-      put_new(acc, k, v)
+    fun = fn element, {acc, seen} ->
+      {key, value} = transform.(element)
+
+      case seen do
+        %{^key => _} -> {acc, seen}
+        _ -> {[{key, value} | acc], Map.put(seen, key, true)}
+      end
     end
 
-    :lists.foldl(fun, [], Enum.reverse(pairs))
+    {result, _seen} = :lists.foldl(fun, {[], %{}}, Enum.reverse(pairs))
+    result
   end
 
   @doc """
@@ -1034,21 +1048,18 @@ defmodule Keyword do
   def merge([], keywords2) when is_list(keywords2), do: keywords2
 
   def merge(keywords1, keywords2) when is_list(keywords1) and is_list(keywords2) do
-    if keyword?(keywords2) do
-      fun = fn
-        {key, _value} when is_atom(key) ->
-          not has_key?(keywords2, key)
-
-        _ ->
-          raise ArgumentError,
-                "expected a keyword list as the first argument, got: #{inspect(keywords1)}"
-      end
+    keys2 = collect_keys!(keywords2)
 
-      :lists.filter(fun, keywords1) ++ keywords2
-    else
-      raise ArgumentError,
-            "expected a keyword list as the second argument, got: #{inspect(keywords2)}"
+    fun = fn
+      {key, _value} when is_atom(key) ->
+        not Map.has_key?(keys2, key)
+
+      _ ->
+        raise ArgumentError,
+              "expected a keyword list as the first argument, got: #{inspect(keywords1)}"
     end
+
+    :lists.filter(fun, keywords1) ++ keywords2
   end
 
   @doc """
@@ -1088,33 +1099,73 @@ defmodule Keyword do
   @spec merge(t, t, (key, value, value -> value)) :: t
   def merge(keywords1, keywords2, fun)
       when is_list(keywords1) and is_list(keywords2) and is_function(fun, 3) do
-    if keyword?(keywords1) do
-      do_merge(keywords2, [], keywords1, keywords1, fun, keywords2)
-    else
+    if not keyword?(keywords1) do
       raise ArgumentError,
             "expected a keyword list as the first argument, got: #{inspect(keywords1)}"
     end
+
+    keys2 = collect_keys!(keywords2)
+
+    {non_matching_rev, keys2, duplicate_keys} =
+      partition_left(keywords1, [], keys2, %{})
+
+    keys2 =
+      Enum.reduce(duplicate_keys, keys2, fn {key, _true}, acc ->
+        Map.update!(acc, key, &:lists.reverse/1)
+      end)
+
+    emitted_rev = emit_right(keywords2, [], keys2, fun)
+    :lists.reverse(non_matching_rev) ++ :lists.reverse(emitted_rev)
   end
 
-  defp do_merge([{key, value2} | tail], acc, rest, original, fun, keywords2) when is_atom(key) do
-    case :lists.keyfind(key, 1, original) do
-      {^key, value1} ->
-        acc = [{key, fun.(key, value1, value2)} | acc]
-        original = :lists.keydelete(key, 1, original)
-        do_merge(tail, acc, delete(rest, key), original, fun, keywords2)
+  defp partition_left([{key, value} | rest], non_matching, keys2, duplicate_keys) do
+    case keys2 do
+      %{^key => []} ->
+        partition_left(rest, non_matching, Map.put(keys2, key, [value]), duplicate_keys)
+
+      %{^key => current} ->
+        partition_left(
+          rest,
+          non_matching,
+          Map.put(keys2, key, [value | current]),
+          Map.put(duplicate_keys, key, true)
+        )
 
-      false ->
-        do_merge(tail, [{key, value2} | acc], rest, original, fun, keywords2)
+      _ ->
+        partition_left(rest, [{key, value} | non_matching], keys2, duplicate_keys)
     end
   end
 
-  defp do_merge([], acc, rest, _original, _fun, _keywords2) do
-    rest ++ :lists.reverse(acc)
+  defp partition_left([], non_matching, keys2, duplicate_keys),
+    do: {non_matching, keys2, duplicate_keys}
+
+  defp emit_right([{key, value2} | rest], emitted, keys2, fun) do
+    case keys2 do
+      %{^key => [value1 | remaining]} ->
+        emit_right(
+          rest,
+          [{key, fun.(key, value1, value2)} | emitted],
+          Map.put(keys2, key, remaining),
+          fun
+        )
+
+      _ ->
+        emit_right(rest, [{key, value2} | emitted], keys2, fun)
+    end
   end
 
-  defp do_merge(_other, _acc, _rest, _original, _fun, keywords2) do
+  defp emit_right([], emitted, _keys2, _fun), do: emitted
+
+  defp collect_keys!(list), do: collect_keys!(list, %{}, list)
+
+  defp collect_keys!([{key, _} | rest], acc, original) when is_atom(key),
+    do: collect_keys!(rest, Map.put(acc, key, []), original)
+
+  defp collect_keys!([], acc, _original), do: acc
+
+  defp collect_keys!(_other, _acc, original) do
     raise ArgumentError,
-          "expected a keyword list as the second argument, got: #{inspect(keywords2)}"
+          "expected a keyword list as the second argument, got: #{inspect(original)}"
   end
 
   @doc """
@@ -1228,18 +1279,21 @@ defmodule Keyword do
   """
   @spec split(t, [key]) :: {t, t}
   def split(keywords, keys) when is_list(keywords) and is_list(keys) do
-    fun = fn {k, v}, {take, drop} ->
-      case k in keys do
-        true -> {[{k, v} | take], drop}
-        false -> {take, [{k, v} | drop]}
-      end
+    predicate = in_keys_check(keys)
+
+    fun = fn pair, {take, drop} ->
+      if predicate.(pair), do: {[pair | take], drop}, else: {take, [pair | drop]}
     end
 
-    acc = {[], []}
-    {take, drop} = :lists.foldl(fun, acc, keywords)
+    {take, drop} = :lists.foldl(fun, {[], []}, keywords)
     {:lists.reverse(take), :lists.reverse(drop)}
   end
 
+  defp in_keys_check(keys) do
+    keys_set = :lists.foldl(fn key, acc -> Map.put(acc, key, true) end, %{}, keys)
+    fn {key, _} -> Map.has_key?(keys_set, key) end
+  end
+
   @doc """
   Splits the `keywords` into two keyword lists according to the given function
   `fun`.
@@ -1295,7 +1349,7 @@ defmodule Keyword do
   """
   @spec take(t, [key]) :: t
   def take(keywords, keys) when is_list(keywords) and is_list(keys) do
-    :lists.filter(fn {k, _} -> :lists.member(k, keys) end, keywords)
+    :lists.filter(in_keys_check(keys), keywords)
   end
 
   @doc """
@@ -1315,7 +1369,8 @@ defmodule Keyword do
   """
   @spec drop(t, [key]) :: t
   def drop(keywords, keys) when is_list(keywords) and is_list(keys) do
-    :lists.filter(fn {k, _} -> k not in keys end, keywords)
+    predicate = in_keys_check(keys)
+    :lists.filter(fn pair -> not predicate.(pair) end, keywords)
   end
 
   @doc """
@@ -1342,12 +1397,18 @@ defmodule Keyword do
   """
   @spec pop(t, key, default) :: {value | default, t}
   def pop(keywords, key, default \\ nil) when is_list(keywords) and is_atom(key) do
-    case fetch(keywords, key) do
-      {:ok, value} -> {value, delete(keywords, key)}
-      :error -> {default, keywords}
-    end
+    do_pop(keywords, key, default, [])
   end
 
+  defp do_pop([{key, value} | tail], key, _default, acc),
+    do: {value, :lists.reverse(acc, delete_key(tail, key))}
+
+  defp do_pop([{_, _} = pair | tail], key, default, acc),
+    do: do_pop(tail, key, default, [pair | acc])
+
+  defp do_pop([], _key, default, acc),
+    do: {default, :lists.reverse(acc)}
+
   @doc """
   Returns the first value for `key` and removes all associated entries in the keyword list,
   raising if `key` is not present.