Happened to me twice. I have no idea how it happens, it surely can't be that difficult for a computer to work out that a room has already been occupied. I've never been on a plane or train and found someone with the same seat reservation.
Why is it, hotels can't seem to work, what I would think were straightforward issues out. For example, I only ever seem to have problems with billing and credit card problems at hotels? Never happens at the Supermarket or the Petrol Station - Only ever at a Hotel
I recently checked into the Holiday Inn Heathrow to be told I had already checked in. I explained that I had only just arrived, I showed them my reservation and they explained they must have another Kettering Northants staying, I said that I thought that was highly unlikely and could they make sure I wasn't going to get billed for 2 rooms on my credit card. I was assured I would not be. Low and behold the following month I get billed for both rooms
DESPITE having only signed one bill!
Similar thing happened at the Intercontinental, Sydney - signed one bill when I left, was charged for another room on my credit card bill - hotel just didn't enter into any communication with me and I had to file a despute with my credit card company that took 4 months to resolve!!