diff --git a/lib/ssh_subsystem_fwup.ex b/lib/ssh_subsystem_fwup.ex
index 0c66191..f8471cb 100644
--- a/lib/ssh_subsystem_fwup.ex
+++ b/lib/ssh_subsystem_fwup.ex
@@ -96,7 +96,8 @@ defmodule SSHSubsystemFwup do
   @impl :ssh_client_channel
   def handle_msg({:ssh_channel_up, channel_id, cm}, state) do
     with {:ok, options} <- precheck(state.options[:precheck_callback], state.options),
-         :ok <- check_devpath(options[:devpath]) do
+         :ok <- check_devpath(options[:devpath]),
+         :ok <- check_fwup_not_running() do
       Logger.debug("ssh_subsystem_fwup: starting fwup")
       fwup = FwupPort.open_port(options)
       {:ok, %{state | id: channel_id, cm: cm, fwup: fwup}}
@@ -210,6 +211,14 @@ defmodule SSHSubsystemFwup do
     end
   end
 
+  defp check_fwup_not_running() do
+    if Enum.any?(Port.list(), &("#{Port.info(&1)[:name]}" =~ ~r/fwup/)) do
+      {:error, "update already in progress"}
+    else
+      :ok
+    end
+  end
+
   defp precheck(nil, options), do: {:ok, options}
 
   defp precheck({m, f, a}, options) do
diff --git a/test/ssh_subsystem_fwup_test.exs b/test/ssh_subsystem_fwup_test.exs
index a525d49..2b38dd0 100644
--- a/test/ssh_subsystem_fwup_test.exs
+++ b/test/ssh_subsystem_fwup_test.exs
@@ -23,7 +23,7 @@ defmodule SSHSubsystemFwupTest do
   def start_sshd(options) do
     {:ok, ref} =
       :ssh.daemon(@port, [
-        {:max_sessions, 1},
+        {:max_sessions, options[:max_sessions] || 1},
         {:user_passwords, [{'user', 'password'}]},
         {:system_dir, 'test/fixtures'},
         {:subsystems, [SSHSubsystemFwup.subsystem_spec(options)]}
@@ -285,4 +285,20 @@ defmodule SSHSubsystemFwupTest do
     # Check that the update was applied
     assert match?(<<"Hello, world!", _::binary()>>, File.read!(options[:devpath]))
   end
+
+  test "does not allow more than one update at a time", context do
+    options = default_options(context.test) |> Keyword.put(:max_sessions, 2)
+    File.touch!(options[:devpath])
+    start_sshd(options)
+
+    large_fw = Fwup.create_firmware(message: :binary.copy("a", 24 * 1024 * 1024))
+    fw_contents = Fwup.create_firmware()
+
+    {:ok, t} = Task.start(fn -> do_ssh(large_fw) end)
+    {output, exit_status} = do_ssh(fw_contents)
+
+    assert exit_status != 0
+    assert output =~ "Error: update already in progress"
+    Process.exit(t, :kill)
+  end
 end