air canada double charging--has this happened to you?

on sunday i bought tickets to go home for christmas for 2 people (1800).
on tuesday, the charge hadn't shown up on my credit card, but looking at the credit available i could see that the money was being held.
on wednesday, my card was declined for being over the limit--the charge had gone through in two parts(900 x2), but there was a third charge equal to both tickets (1800) attempting to go through--and as it was over my limit, it was holding all my available credit.
air canada denied that they had put through 2 x 900, claiming they had successfully put through 1800 x 1. the bank refused to cancel it, and it's only now, saturday morning, that it has finally come off.

is this normal?