Christ is condemning vanity - praying to show off.
Vain means to do something out of pride. The Pharisees were experts at praying so that their piety could be seen. They were showoffs.
*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. (Matthew 6:5)
They do all their deeds to be seen by men; for they make their phylacteries broad and their fringes long, (Matthew 23:5)*
Christ is condemning praying out of vanity, praying to show others how pious you are.
Christ himself used repetative prayer
*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.” (Matthew 26:39)
Again, for the second time, he went away and prayed, “My Father, if this cannot pass unless I drink it, thy will be done.” (Matthew 26:42)
So, leaving them again, he went away and prayed for the third time, saying the same words. (Matthew 26:44)*
Jesus always withdrew to pray - to the desert, to a lonely place, away from the disciples in the Garden of Gethsemani. Christ did not pray vainly, but in secret. He himself however, used repetative prayer.
If a Catholic wants to go someplace private like a Church or Chapel or their own home and pray a Rosary then it is no ones business but their own and they are imitating Christ who withdrew and prayed repetatively. We are not supposed to pray the rosary on the street corner to show off.
-Tim-