i encounterd a problem when adding a Facebook Pixel tag. Without any modification and a mapped Pixel ID the following warning is generated "[Facebook Pixel] - Duplicate Pixel ID: XXXXXXXXX"
Taking a look into the the tag template the line fbq("init", ... is called twice.
Once in the else clause of the u.initialized if-statement line 442 and once within the loader_cb function in line 324.
Thanks for pointing this out - this has been highlighted, and should be fixed in a future release for the tag.
Be assured that the warning in the console should have no impact, and should not cause any problems with your tracking / cause no duplicate events. When initialising twice, The FB pixel recognises this and sends out the warning.