How to fix Duplicate Facebook pixel ID error?

Another answer found in the Google Forum: "You have to check whether your Facebook Pixel code is only initialised once per page (and not per event). Go to the tag where your FB Pixel code is and open the advanced settings. Tag firing options -> Once per page (not event!)."

I found the solution on this page actually.

Its because everytime you send an event the code is initialized with the same pixel id. Actually initialization should happen only once."

This fix works. You can verify that by using the Facebook Pixel Helper as an extension in Chrome.

if(typeof fbq === 'undefined') {
    !function(f,b,e,v,n,t,s){if(f.fbq)return;n=f.fbq=function(){n.callMethod?
    n.callMethod.apply(n,arguments):n.queue.push(arguments)};if(!f._fbq)f._fbq=n;
    n.push=n;n.loaded=!0;n.version='2.0';n.queue=[];t=b.createElement(e);t.async=!0;
    t.src=v;s=b.getElementsByTagName(e)[0];s.parentNode.insertBefore(t,s)}(window,
    document,'script','https://connect.facebook.net/en_US/fbevents.js');

    fbq('init', '123123123213121');
    fbq('track', 'PageView');
    fbq('track', '{{fBPixelType}}');
  }
  else {
    fbq('track', '{{fBPixelType}}');
  }

This error generally means fbq('init', <pixel_id>) is getting called more than once with the same pixel id.