Hi HT, because they aren't "duplicates"
There is actually quite a bit in each Gospel that is unique. And I believe all four Gospels approach the same "Subject", Jesus Christ, from four different POV's (as "King" in St. Matthew, as a "Servant" in St. Mark, as the "Man" in St. Luke, and as "God" in St. John).
Are you just beginning to read through the Bible for the first time? If so, you may find it more interesting to skip back and forth between the Gospels and the Epistles rather than reading the NT straight through (though straight through isn't bad either
). And if you've not read the Gospel of John yet, that's where I'd recommend you go next
Yours in Christ,
David