title: on_command_error repeating message
body: <p>I have this event to catch errors</p>
  
  <pre class="lang-py prettyprint-override"><code>@bot.event
  async def on_command_error(msg,error):
      if error.args[0] == 'The check functions for command hi failed.':
          await msg.send("{} **is not an NSFW channel**".format(msg.channel.mention))
  
  @commands.is_nsfw()
  @bot.command()
  async def hi(msg):
      msg.send("OK")
  </code></pre>
  
  <p>When the event catches the error it keeps on sending the same message twice.
  For example it would send <strong>#channel_name is not an NSFW channel</strong></p>
  
  <p>I'm using the rewrite branch of the discord.py</p>
  
  <p>Thanks for the help in advance</p>