It was not a boon from Karna to Kunti but a promise.
Karna promises Kunti that he shall only slay Arjuna in the battle even he has a chance to slay Yudhistira and others. Using the same astra twice is not at all mentioned in the conversation of Karna and Kunti.
For the sake of Dhritarashtra's son, I shall fight against thy sons to the best of my strength and might. I must not, however, abandon kindness and the conduct that becometh the good. Thy words, therefore, however beneficial cannot be obeyed by me now. This thy solicitation to me will not yet be fruitless. Except Arjuna, thy other sons, Yudhishthira, Bhima, and the twins, though capable of being withstood by me in tight and capable also of being
slain, shall not yet be slain by me. It is with Arjuna alone, among all the combatants of Yudhishthira, that I will fight. Slaying Arjuna in battle, I shall achieve great merit, or slain by Savyasachin, I shall be covered with glory. O famous lady, the number of thy sons will never be less than five. Five it will always be,--either with me, or with Arjuna, and myself slain.'
Udyoga Parva section 46
Hearing those words from Karna, Kunti was trembling with grief and embraced her son and said these words :
'Indeed, O Karna, even if what thou sayest seemeth to be possible, the Kauravas will certainly be exterminated. Destiny is all. Thou hast, however, O grinder of foes, granted to four of thy brothers the pledge of safety. Let that pledge be borne in thy remembrance at the time of shooting of weapons in battle.' And having told all this, Pritha also addressed Karna, saying, 'Blessed be thou, and let health be thine.'
But during the battle with Arjuna, Karna refuses to use the same shaft twice. This is not a part of the promise to anyone but his morality not to use a failed arrow twice. When a snake asks to shoot the same shaft twice against arjuna, he refuses.
Karna said, "Karna, O snake, never desires to have victory in battle today by relying on another's might. Even if I have to slay a hundred Arjunas, I will not, O snake, still shoot the same shaft twice." Once more addressing him in the midst of battle, that best of men, viz., Surya's son, Karna, said, "Aided by the nature of my other snaky weapons, and by resolute effort and wrath, I shall slay Partha. Be thou happy and go elsewhere."
Karna Parva section 91
So Kunti neither asked for a promise about using same shaft twice nor Karna granted it.