Matthew 6:7: "And in praying do not heap up empty phrases as the Gentiles do; for they think that they will be heard for their many words."
In this verse, Christ condemns prayers that fall into the category of "vain repetition". We can be sure that he is not forbidding repetitious prayers as such, because he himself prayed repetitiously, and gave us the greatest of all prayers, the Our Father, which he intended us to pray repeatedly. God the Holy Spirit inspired many repetitious prayers in Scripture, intending that they be prayed and sung frequently by believers. Finally, we should notice that in the Garden of Gethsemane Christ repeated the same prayer three times during his agony. - Matthew 26:39, 42, 44: "And going a little farther he fell on his face and prayed, 'My Father, if it be possible, let this cup pass from me; nevertheless, not as I will, but as you will.' . . . Again, for the second time, he went away and prayed, 'My Father, if this cannot pass unless I drink it, your will be done.' . . . So leaving them again, he went away and prayed for the third time, saying the same words."
Catholics are criticised for praying the Rosary - the repetitious series of scriptural prayers that help us meditate on key episodes of the Gospels, as well as mysteries of the life of the Blessed Virgin Mary.
With regard to the Rosary, we should recall that Mary is the Ark of the New Covenant, especially chosen by God to be the mother of Christ, the theotokos (which in Greek means "God-bearer"). One of the most important ways Mary was prefigured in the Old Testament was the Ark of the Covenant (Exodus 25; 2 Samuel 6). The Ark of the Covenant was used by the Israelites in battle against the enemies of the Lord. It brought down the walls of Jericho (Joshua 6); its holy presence routed the Philistines in more one than one battle (Numbers 10:35). In a similar way, we Christians pray the Rosary and ask Mary, the Ark of the New Covenant, to intercede on our behalf and use her prayers as a weapon of grace against the evil one. The evil one is spoken of in Revelation 12 as the Red Dragon, who does battle with the "woman", and when we pray the Rosary we go well-armed into battle against the adversary, who is "prowling round like a roaring lion, looking for someone to devour" (1 Peter 5:8).
Psalm 136, inspired by the Holy Spirit, is a good example of repetitious prayer. The phrase "For his steadfast love endures forever" is repeated many times.
Notice how in the passage from Daniel, there is a repetition of "Bless the Lord" at the end of each verse, much as Catholics pray a litany, a very biblical practice.
One word of warning, though. We should never rattle off prayers like a machine gun, either at Mass, in our own prayer time, or when we pray the Rosary. We should consider carefully what we are saying, and who we are speaking to. We would not dream of being so appallingly rude as to speak to another person in a fast, unthinking way, and we certainly should not do that to God!