Dear Rabbi, Is it permitted to repeat words in davening which are part of a song, e.g Lechi dodi or hashivenu hashem, when returning the sefer torah to the Aron?

ב"ה Shalom The overall direction of the poskim is not to repeat words in Tefiila for the sake of the melody especially when repeating it includes Hashem's name. Among the reasons given are is that the repetition is considered an interruption and if one repeats Hashem's name even when saying an entire sentence it seems that the persons is praying to two deitiesח"ו. (שו"ת מהר"ם שיק סי' לא, ערוך השולחן סי' שלח ס"ק ח, אגרות משה או"ח ח"ב סי' כב, יביע אומר ח"ו או"ח סי' ז אות ד ) ) However, there are parts of the Tefilla where the repetition is an integral part of the Tefilla and the common custom. In these instances it is permitted. Some say, that if it is not part of Kriat Shema, Shmoneh Esreh, or in parts of the Tefilla where the name of Hashem is mentioned, it is permitted. (See שו"ת חבל נחלתו יב ג and references ad. loc.). Therefore, in each Bet Knesset the Rav should be consulted . All the best.