Janisj gives good advice, flsd. I'd recommend doing the same.
Otherwise... some salt, pepper, a dash of mustard, and enjoy. ;)

I would dispute that one, also, but it isn't just that you made a mistake and booked twice (in which case, I would say, nice if they refunded it but really your fault). However, in this case you say their website was faulty and not showing your first booking. So that would be their problem.

Thanks for the advice - I will take it up with my credit card. I realized it was a nonrefundable reservation, but as I said, wouldn't have made the second reservation had booking.com ever acknowledged the existence of the first one or had it appear when I clicked on "my reservations" under my account at their site. This is one time I believe the booking service and/or the hotel should issue a refund. Thanks again for the good advice.

UPDATE: I wrote a humble and courteous note to Colour Hotel asking them to waive the non-refundable provision and, please, not to charge me for the second reservation/cancellation, and they essentially responded "go eff yourself." I guess customer service isn't important in Deutschland.