Walter Montego: I think that in the US, a double room is a room with two double beds, while in the UK it is a room with one double bed.

The reason why I was asking: I was told that the omission of “a” in such sentences is "one of those strange and commonly accepted uses of articles." Now that I've read your responses, it doesn't seem that it really is such a commonly accepted use. :) Thank you all.