User blog comment:Miskos3/Fairy Tail Episode 249: Tartaros arc: The Celestial King vs. The Underworld King, Review/@comment-180.191.130.34-20150906035140/@comment-4933386-20150906105537

If Layla summoned Celestial Spirit King in front of Mard Geer, Mard Geer would've recognized Lucy when she summoned CSK or something

But that theory is pretty interesting. But I do think that CSK have been summoned once other than Lucy, and it may as well be Layla.