Regarding repetition in prayer:
Revelation 4:8 says of the four living creatures around God's throne in the heavens: (NRSV)
"Day and night without ceasing they sing, "Holy, holy, holy, the Lord God the Almighty, who was and is and is to come."
Not only that, but the 10th and 11th verses indicate the use of responsive prayer:
"the twenty-four elders fall before the one who is seated on the throne and worship the one who lives forever and ever; they cast their crowns before the throne, singing, "You are worthy, our Lord and God, to receive glory and honor and power, for you created all things, and by your will they existed and were created.""
Also, from my personal standpoint, I find that saying the "Our Father" or reading one of the Psalms (yes, they are prayers too) at the start of my prayer puts my mind and heart in communion with Holy matters. When I do this, I find that the remainder of my prayer for my own intentions is much more meaningful.
BTW, I was raised as a Witness in the 1950s. I learned the Our Father prayer at my mother's knee -- from a King James Version Bible. And when I was in 1st grade, my homeroom teacher had the entire class recite the Lord's Prayer first thing in the morning. My parents allowed me to participate so long as I did not add the part at the end that Witnesses believe was not original. In 2nd grade, my teacher had us recite Psalms 23 first thing in the morning, and I was allowed to do that also. I did not participate in the flag salute, but I DID pray in public.
I doubt that Witnesses today are permitted to do these things -- but life was different away back when. If it came directly from the Bible, it was okay no matter who was reading it or saying it.
NanaR