Are there supplications in the Qur’an and Sunnah that a person afflicted with stuttering can recite to solve the problem of difficulty in speaking and the repetition of letters?

The tribulation of a believer through illness atones for sins and elevates ranks. Allah tests His servants to reveal those who are patient and seek His reward. Tribulation may be a punishment, but it becomes expiation if the servant is patient and seeks reward from Allah. There is no specific supplication in the Sunnah for being afflicted with this particular illness, and there is generally no cure for it in modern medicine. Indeed, it is not an illness but rather a challenge that requires self-confidence, a supportive environment, speech training, and the assistance of a specialist.