The Operative Word is "Vain" not Repetition
Lawrence Gerard Fox
Completed Licentiate in Sacred Theology (STL) on June 12, 2021 (Magna Cum Laude), International Theological Institute, Austria. Living in Texas.
After reading through the Psalms (over the years), I no longer worry myself about such things as "repetition" in prayer.
As for the notion of "vain" repetition, the concern is not over repetition but not allowing the LORD to speak at anytime before, during and after such repetition.
Vain means empty, without substance (without essence) and in the case of prayer, or oaths or promises, the intent to call upon the name of the LORD in a half-hearted or no-hearted manner is vain. "You shall not take the name of the LORD your God in vain, for the LORD will not leave anyone unpunished who takes His name in vain" (Cf. Exodus 20:7).
So what about one's every day life of conversation with the LORD?
The key -- to avoid the notion of "vain" -- would be "to intend" union (even a brief union) with the LORD, "to minimize distractions" and "to collect" from what has been read, spoken, and heard.
Be that as it may, as it relates to "repetition", I do not worry about such things since repetition is all over the Psalms and I repeat them weekly and sometimes daily.