The way I'm understanding the ruling is that having the bane condition with the text "if you would gain a Boon Condition you may discard this card instead" supersedes the rule that you can't accept a boon if you already have that one. Taking this into consideration that they don't want you to be doubling up on boons but they will let you discard a bane at the sacrifice of the boon even if it is a duplicate because the boon is discarded with the bane.
Look at it as this, your Boon is the center of a Tootsie Roll Pop and the Bane is the hard candy shell that surrounds the center. Mateo's Boon which is a duplicate has to penetrate the hard candy shell Bane first, before its able to get to the center of the Tootsie Roll Pop (your Boon). At this point it will have already been discarded before it would have reaches your boon the center of the Tootsie Roll Pop.