But God only said it once...

The phrase “new covenant” only appears in the Old Testament in Jer 31:31. This is clearly a classic case of the importance of weighing items rather than counting them when it comes to determining significance.— The Death of the Messiah and the Birth of the New Covenant, page 24

<idle musing>
Maybe he only needed to say it once because he figured we would be listening : ) Seriously, though, Gorman makes a good point here. Just as you don't count the number of manuscripts to decide a reading in textual criticism, you shouldn't count the number of occurrences of a word/concept in scripture either.
</idle musing>