You are charged stamp duty per credit card *account*, but you are charged stamp duty per ATM *card*.
Getting a card in a second name on a credit card account is pretty normal, no big deal, and happens all the time. I presume most married people operate with this type of setup - except for the ones who insist on maintaining seperate bank accounts.
It is pretty obvious to me, but as stated above the account is in one persons name only, so be absolutely sure you understand the spending patterns and ability to repay of the other person before getting a card in somone elses name on your account. You will be the one in debt if they overspend on your account and all the small and not so small print on the forms will ensure that you can't tell the bank "I didn't know they could spend my money . . ."
In answer to the original question, the bank was wrong and your friend was right.
z