Is a person to be judged an infidel (kafir), required to perform ghusl, and to repeat the Shahada if he told a Muslim, "You are an infidel," and then claimed he was joking, knowing that he had previously been informed of the hadith, "Whoever says to his brother, 'O infidel,' then it has surely fallen upon one of them"?

The hadith you referred to was narrated by Al-Bukhari and Muslim, on the authority of Ibn 'Umar, with the wording: "Any man who says to his brother: 'O disbeliever,' then one of them has indeed incurred it." Muslim added in a narration: "If he was as he said; otherwise, it would revert to him." In another wording by Muslim: "If a man declares his brother a disbeliever, then one of them has indeed incurred it."