I'm thinking that somewhere in the T&Cs it says you can only sign up once. Presumably the worst that would happen would be to pay the difference.
The worst that happens for HotelClub is they get heaps of frequent travellers signed up to their website, all of which can be emailed regularly. I admit to already using the $50 for an upcoming Perth trip.
One feature that worked for them. I did a booking process up to payment stage just to check out availability and final pricing but didn't complete the card details. Then cancelled. To their "programming" credit, 24 hours later I received a detailed email to remind me I didn't have a reservation and with a link to continue the booking. It was very easy to then complete it.
I've looked at other hotels on HotelClub but without the $50 credit I have found they are not competitive with other online booking engines. ie Wotif.
...now how to spend Mrs Albatross USD50 credit....
Alby