If you get an "(Plug.Conn.AlreadySentError) the response was already sent" error:
```elixir
Server: localhost:4000 (http)
Request: POST /api/v1/xxx
** (exit) an exception was raised:
** (Plug.Conn.AlreadySentError) the response was already sent
(plug) lib/plug/conn.ex:332: Plug.Conn.put_status/2
```
Verify that you have not defined the action plug more than once:
```
plug :action
```