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diff --git a/lib/pleroma/web/plugs/idempotency_plug.ex b/lib/pleroma/web/plugs/idempotency_plug.ex
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+# Pleroma: A lightweight social networking server
+# Copyright © 2017-2020 Pleroma Authors <https://pleroma.social/>
+# SPDX-License-Identifier: AGPL-3.0-only
+
+defmodule Pleroma.Web.Plugs.IdempotencyPlug do
+  import Phoenix.Controller, only: [json: 2]
+  import Plug.Conn
+
+  @behaviour Plug
+
+  @impl true
+  def init(opts), do: opts
+
+  # Sending idempotency keys in `GET` and `DELETE` requests has no effect
+  # and should be avoided, as these requests are idempotent by definition.
+
+  @impl true
+  def call(%{method: method} = conn, _) when method in ["POST", "PUT", "PATCH"] do
+    case get_req_header(conn, "idempotency-key") do
+      [key] -> process_request(conn, key)
+      _ -> conn
+    end
+  end
+
+  def call(conn, _), do: conn
+
+  def process_request(conn, key) do
+    case Cachex.get(:idempotency_cache, key) do
+      {:ok, nil} ->
+        cache_resposnse(conn, key)
+
+      {:ok, record} ->
+        send_cached(conn, key, record)
+
+      {atom, message} when atom in [:ignore, :error] ->
+        render_error(conn, message)
+    end
+  end
+
+  defp cache_resposnse(conn, key) do
+    register_before_send(conn, fn conn ->
+      [request_id] = get_resp_header(conn, "x-request-id")
+      content_type = get_content_type(conn)
+
+      record = {request_id, content_type, conn.status, conn.resp_body}
+      {:ok, _} = Cachex.put(:idempotency_cache, key, record)
+
+      conn
+      |> put_resp_header("idempotency-key", key)
+      |> put_resp_header("x-original-request-id", request_id)
+    end)
+  end
+
+  defp send_cached(conn, key, record) do
+    {request_id, content_type, status, body} = record
+
+    conn
+    |> put_resp_header("idempotency-key", key)
+    |> put_resp_header("idempotent-replayed", "true")
+    |> put_resp_header("x-original-request-id", request_id)
+    |> put_resp_content_type(content_type)
+    |> send_resp(status, body)
+    |> halt()
+  end
+
+  defp render_error(conn, message) do
+    conn
+    |> put_status(:unprocessable_entity)
+    |> json(%{error: message})
+    |> halt()
+  end
+
+  defp get_content_type(conn) do
+    [content_type] = get_resp_header(conn, "content-type")
+
+    if String.contains?(content_type, ";") do
+      content_type
+      |> String.split(";")
+      |> hd()
+    else
+      content_type
+    end
+  end
+end