Thank you all for your replies so far.

The ticket numbers for the two tickets are different. The credit card charge from December matched the ticket number that my father actually traveled on. The credit card charge from February (the one we are disputing) had a ticket number that nobody recognized.

Both the December charge and the February charge showed Winnipeg as the location of the charge. I suppose that is where Air Canada processes their credit card charges.

The signature on the slip Air Canada provided appears to be my father's signature.

I have a new theory as to where that credit card slip came from. My father purchased a ticket in Feb 2002 for the same itinerary. I will ask my dad to check the ticket number for that ticket. After all the bad luck he's had with Air Canada last year, it would not surprise me if he got double charged in Feb 2003 for a ticket he bought and traveled in Feb 2002. I will keep you all posted.