In Matthew 19:9, I can't find the phrase "and whoever marries her who is divorced commits adultery" in my translation.
"And I say to you, whoever divorces his wife, except for sexual immorality, and marries another, commits adultery; and whoever marries her who is divorced commits adultery" (Matthew 19:9 NKJV).
"And I say to you, whoever divorces his wife, except for immorality, and marries another woman commits adultery" (Matthew 19:9 NASB95).
The New Testament has numerous manuscripts of varying quality. The manuscripts were hand copied from older copies and sometimes mistakes crept into the text. Of course, once a mistake appears in an old text and it becomes the base text from with other texts are copied, the mistake gets replicated. There is a science to studying how people tend to make copy errors and by examining the various family of texts, scholars try to reverse the mistakes.
In this case, the last phrase: "and whoever marries her who is divorced commits adultery" is not in all the manuscripts. Supporting in the phrase are manuscripts 017K, 28, 700, 892, the Byzantine family of texts, and the lectionaries. Without the phrase are manuscripts 01א, 04C-corrected, 05D, 019L, 1241. Because older manuscripts are missing the phrase, the assumption is that this was either the accidental insertion of someone's marginal notes or some scribe thought he was fixing an error by making Matthew 19:9 read like Matthew 5:32.
The King James Version used the Byzantine family of texts for the most part and the New King James Version follows the same base text with a few corrections. Most newer translations use a blend of texts usually based on the United Bible Society. The United Bible Society tends to favor the older texts.
Even though it is debated whether this phrase should be in Matthew 19:9, there is no similar debate about the phrase in Matthew 5:32. Thus, whether the phrase is included in Matthew 19:9 or not, it makes no difference in our understanding of God's teachings on divorce.