There is an argument that any significant T&Cs, rather than the several pages of legalese in 6 point font, that might affect a person's decision to make the purchase, should be pointed out to the buyer in a 'key facts' type arrangement - this would cover a 100% cancellation fee because that, in effect, makes the ticket non-refundable, so changes the terms of the offer presented.
When DH was booking, did it suggest that the booking was refundable? Is this a case of the information being deliberately deceptive or has he not read it properly?
As an aside, for those who dismiss the mental load, it is this sort of thing that I mean when there's a difference between just doing things, and doing them well. Have you got the best price, are the T&Cs as you expected, have you actually booked the right day/time/location etc. It's well known that tickets for anything are often non-refundable, or so expensive to refund that it's not worth doing, so never assume you can cancel something without penalty. There's lots of stories in this thread where 'DH booked the wrong XYZ' so doing that due to carelessness or lack of thorough reading is just as bad as not doing it at all.
But in any case, I would ring and say 'accidentally booked twice', is it possible to refund. If yes, great. If not, try emailing the same question via customer service. Good luck.