Alternate reality
Lieutenant Hannity was an operations division officer assigned to the USS Enterprise in 2258 of an alternate reality.
With the ship on approach to Vulcan, and James T. Kirk warning of an attack, Captain Christopher Pike ordered her to hail the USS Truman. Hannity reported that the fleet had arrived at Vulcan, but all contact had been lost. (Star Trek)
Hannity was played by Amanda Foreman, who later played Brackett.
The Star Trek Encyclopedia had entries for both "Ensign Brackett" (4th ed., vol. 1, p. 100) and "Hannity" (4th ed., vol. 1, p. 325). It is clear in the entries and the notes accompanying the entries that the authors of this reference work believed the actress was playing two characters, not one.