Is repeating the same prayer all the time a sin or angering God?

First, this question asks for an interpretation, since the Bible never says "don't repeat this prayer." There are certain prayers that we are told are not heard by God, such as in 1Kings 18:21, when the priests of Ba'al cried out repeatedly to their pagan deities but they received no reply, and the Prophet Elijah mocks them for worshiping idols. But mostly, the Hebrew Bible only asks that worshipers pray with sincerity and humble themselves before God. Also, we know that certain prayers were in fact repeated, and they may even have been chanted. But the commentaries on scripture by sages and scholars warned against just repeating words over and over with no thought of what was being said. The key is not the number of times you repeat it (after all, Catholics do repeat the prayers in the Rosary, to cite one example), but whether you are sincere and truly attempting to show your devotion to God when you pray.

another answer:

The Bible does not always say after each command: "It's a sin. It angers God." It just gives the instructions. If we can't speak from our heart; what does it say about our relationship with God? Jesus specifically said not to vainly repeat the same prayer over and over again:- Here is the verse from Matthew 6:7, in several translations:

And when you pray, do not keep on babbling like pagans, for they think they will be heard because of their many words. (New Living Translation)

But when ye pray, use not vain repetitions, as the heathen do: for they think that they shall be heard for their much speaking. (King James Version)

Some translations use "like the hypocrites" rather than "like the heathens." This refers to praying with meaningless words, or useless repetition. Such foolish prayer is often the result of heathenism, hypocrisy or ignorance.