Right now (26/11/2015, UTC 17:16), British Airways is flying over the Australian Outback with two Boeing 777-336(ER)s, christened flight BA15 (http://www.flightradar24.com/BAW15/8169bb5) and BA15X, which is slightly ahead (http://www.flightradar24.com/BAW15X/8169237).
Why is this? Is flight BA15X a logistic one? And if not, why did they use an "X" to distinguish between the two instead of another number?
I am also wondering: is this going to cost extra money to
BA, owing to the new slot they had to purchase or rent for operating a contemporary, second aircraft at