Q. Is there any problem with cantors repeating words and phrases for musical emphasis?
A. The Shulchan Aruch (Orach Chayyim 53:11) is critical of those who prolong or embellish prayers to impress or entertain the congregation. When repeating words and phrases became common, this was opposed by one authority after another (see Rav Moshe Feinstein, Igg’rot Moshe, Orach Chayyim 2:22).
Rabbi Ovadiah Yosef (Or Torah, Kislev 3737) says that such repetition affects the solemnity of the service and tends to turn it into a concert, can distort the meaning of the Hebrew text, and can constitute a hefsek (interruption) to a b’rachah.
However, repeating of Biblical verses may be allowed, e.g. during Hallel it is usual to repeat Psalm 118:25, etc. This does not distort the text or create a hefsek, and may indeed add to the praise of God.