This whole situation can lead to embarrassing times! From what I've learned:

1. "Most" hotels charge block your anticipated charges (# of nites X rate + taxes) PLUS 25% of that total.

2. One way to eliminate your card being charged twice -- ask the desk clerk to call in the credit (it will get posted immediately -- usually!) or... call it in yourself. Our credit card companies (US ones) look at the record, see the two charges and put some sort of code into the system to put the "block" charge back into available credit.

And Tom T -- do a couple of hotels in a week and a couple of car rentals (they tend to do the same "block" procedure) and YES! it's easy to blow the credit limit!