What is the ruling on praying behind an Imam who repeats a letter from the letters of Al-Fatihah, such as saying: "ولا الضا" then stops and returns to the letter Dhaad, then continues?

It is disliked to be led in prayer by someone who repeats a letter from Al-Fatiha without changing the meaning. However, if the meaning is changed, even by substitution, a non-standard recitation (qira'ah shadhdhah), or an alteration of meaning, then if the person is unable or ignorant, their prayer is valid. Repeating the letter dad in "Al-Dallin," which originally involves gemination (tashdid), does not invalidate the prayer. It is preferable to replace the imam if someone more proficient in recitation can be found.