Recently, I ordered a M/C for my 17 year old daughter and hung it on to her Alaska account, in order to keep her miles active.

Subsequently, my M/C Idines were showing up on her account. I did not notice that her credit card account # is the same as mine (not so for Amex, btw), and that by adding her card, Idine automatically disabled my M/C card with the same number. Frankly, I’m surprised that I didn’t get a, “this card is already registered on a different program” kind of message.