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Caldwell previously has sung with former Voice coach Christina Aguilera, who won The Voice last season. Caldwell's website explains: Ali Caldwell recently had the luxury of performing as a supporting vocalist for the amazing talent Christina Aguilera during her recent record-breaking concert. This took place at the second largest music festival in the world, the Mawazine Rabat Festival in Morocco, Africa. She performed to the largest audience for a female performer in history, with an astounding 250,000 people in attendance. (Instagram)