Sorry. I didn't get notice of your comment... What I meant was that they wouldn't let me re-use the 11-digit number that's on my 1K card. The issue was not that I'd used my 16-digit credit card number before, in the UA or any other dining program. Upon completing the Enrollment page, and clicking "continue>>", I get it again with this in red:
! ERROR: The Mileage PlusŪ Number you selected has already been used. Please try again.
I've never enrolled, or used it (or either of my credit cards), in Mileage Plus Dining before.