I want my bot to respond once to a command such as .on. However, it responds multiple times per input:

The code is:

client.on('message', message =>{
    if(message.content === '.on'){
        message.channel.sendMessage('Testing Bot is now Online, Greetings, ' + message.author.username);
    }

If anyone could point me in the right direction to make the bot respond once that would be great.