Fortunately important things happened in a very dry climate, so we now have copies (and translations) of the Gospel of Peter, Gospel of Thomas, and other early works that were suppressed, but copies were expensive, so they just hid them, and 1700 years later they turn up.
So when counting sources, would you count a book suppressed as heresy because it had theological error, or not count it despite it having similar accounts to other books that were not suppressed?
Or since sources could copy each other, do you discount all but the earliest?
Nobody said it was easy. In fact, just about everybody says it is incredibly hard.