First, here is the passage in context:
Mt 6:1-8 "Beware of practicing your piety before men in order to be seen by them; for then you will have no reward from your Father who is in heaven. 2 "Thus, when you give alms, sound no trumpet before you, as the hypocrites do in the synagogues and in the streets, that they may be praised by men. Truly, I say to you, they have received their reward. 3 But when you give alms, do not let your left hand know what your right hand is doing, 4 so that your alms may be in secret; and your Father who sees in secret will reward you. 5 "And when you pray, you must not be like the hypocrites; for they love to stand and pray in the synagogues and at the street corners, that they may be seen by men. Truly, I say to you, they have received their reward. 6 But when you pray, go into your room and shut the door and pray to your Father who is in secret; and your Father who sees in secret will reward you. 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. 8 Do not be like them, for your Father knows what you need before you ask him.Protestants may be more familiar with the KJV rendering of vs. 7: "But when ye pray , use not vain repetitions, as the heathen do: for they think that they shall be heard for their much speaking." At any rate, the context is crucial here. What this reveals is that Jesus was not condemning repetition as such, but vain repetition, that is, saying words over and over so that you can be seen and heard by men and receive their praise. Jesus is speaking against those people who pray, not out of humility and adoration of God, but so that they can be known for their so-called piety. Catholics don’t pray the rosary with that intention, thus this passage does not apply.
If Jesus was condemning all forms of repetition in prayer, then we must charge Jesus Himself with praying incorrectly. After all, He repeated the same prayer three times in the Garden of Gethsemane:
Mt 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 thou wilt." [...] 42 Again, for the second time, he went away and prayed, "My Father, if this cannot pass unless I drink it, thy will be done." [...] 44 So, leaving them again, he went away and prayed for the third time, saying the same words.Ever read the Psalms? They are filled with repetitive prayer. Psalm 136 repeats the phrase “his steadfast love endures forever” 26 times! Worship in heaven is also repetitious: “day and night they never cease to sing, ‘Holy, holy, holy, is the Lord God Almighty, who was and is and is to come!’” (Rev 4.8).
It's clear from this that, when it comes to praying, the intentions of the heart are what matter most.