Originally Posted by
Cledaybuck
If you don't want to take that chance, just don't put your AA FF# on the flight.
Since it will be booked with BA (afaik AA won't sell you a cash ticket LHR-BCN), they only pull the FF# for purposes of status (for benefits) - they only send the information over for credits after the flight has been flown. So yes, AA will know about it eventually, but I wouldn't worry as it won't appear as a duplicate reservation.