Has some of the New Testament been plagiarized?

Repeating an earlier aphorism or slogan is not plagiarism unless one claims that it original. For example, John the Baptist declared "Repent for the Kingdom of Heaven is at Hand" (Luke 3:1) prior to Jesus taking up this slogan. In this respect, these were simply two teachers who taught the same message. Likewise, Hillel's summary of the Law had apparently gained widespread popularity a generation prior to Jesus repeating it. We should be prepared to admit that this teaching was not original to Jesus, but the fact that Jesus taught the same thing that Hillel did does not constitute plagiarism. Jesus agreed with Hillel on many issues, from his emphasis on following the spirit of the law rather than its letter to his attitude toward interacting with Gentiles. ("What is hateful to you, do not do to your fellow man" is Hillel's famous reply to a Gentile seeker who had been rejected by Hillel's prominent opponent, Shammai.)

With regard to the teaching of other religious leaders and philosophers, this is best understood by seeing them as teaching a universal truth, rather than consciously borrowing from one another.

NOTE: By the way, some followers of John the Baptist apparently did object to Jesus' borrowing from John. The Mandaeans of Iraq (few remain there today) see themselves as followers of John's teaching, and they consider Jesus to have been an apostate disciple of the Baptist.

The OP has documented a few of the many, many times the Bible quotes other sources. here are a few more. I personally have a more complete list published in an appendix of "the Comprehensive New Testament" which lists thousands of such quotes/allusions throughout the NT.

Bible Quotes and Allusions from Non-Biblical Sources.

In the case of Matt 7:12, "The Golden Rule", the above reference lists many of those posted by the OP and several others. However, most of these stated the rule in a negative way - Christ turned this around and stated it in the now famous positive sense.

In the case of the "Beginning and End/First and Last" statements found in the book of Revelation 1:8 (and V17), 2:8, 22:13, these appear to be a quotation from the OT in Isa 41:4, 44:6, 48:12. Interestingly, this title is applied to YHWH in the OT and to Jesus in the NT. However, similar titles were used in other writing but all were written well after the book of Isaiah. This suggests that the other sources copied Isaiah and the Book of Revelation alludes to Isaiah.

Answer

It could be very difficult to state who plagiarized whom. Great chances are there that others plagiarized from the New Testament!

Explanation

Of all the ancient documents, the New Testament is the best attested as:

1. internally consistent among different epistles and gospels on one hand and with the Old Testament on the other;

2. externally affirmed by the earliest church fathers;

3. written within a single generation so that there was no time for “legends” (courtesy C S Lewis an earlier long time atheist) to be created on them (example, the earliest copy of John’s gospel written within 30 to 50 years of his death whereas many of the earliest copies of ancient authors are between 500 years (Homer’s Iliad) to 1300 years (Herodotus and others);

4. written during the lifetime of most of the eye witnesses and enemies who could verify or oppose; and

5. having the maximum number of manuscripts (Greek alone more than 5000 and still increasing) and together with ancient translations more than 25000! Other ancient documents are nowhere close by).

Besides, skeptics have scrutinized the manuscripts of the NT so hard so that the supporters confirmed the accuracy of it to be 99.5% or more while that of the other documents are mostly on shaky ground.

A Word of Caution

There was a time when I tried to study whether Jesus of Nazareth was a copy of lords Mithra, Horus, Budha, Krishna and others.

I learned that these other figures could have been real historical figures to begin with but many centuries lapsed before finally a comprehensive story was developed. But during these centuries several legendary accounts were created.

The best example is Lord Buddha. (I would have been a Buddhist if I were not a Christian). There was a historical figure who lived long before Jesus of Nazareth. But most of the stories revolving around him were created through several centuries when finally things were written down after a millennium.

So we do not know who plagiarized whom!

I also learned that many of the manuscripts of these stories were written long after the New Testament books.

I was also pleasantly surprised to learn from C S Lewis that in the Bible, often, the miracles are underplayed (mostly written in one or two sentences) as less important than the teachings. This may be because, to God, as the real author, miracles are “normal” business whereas in other legendary stories, miracles are often “overplayed” with large superlative descriptions used to narrate the same.

Conclusion

An extensive and intensive study is required to really see who copied whom with the earliest manuscripts and the actual history scrutinized.

Until then I would like to keep myself open to critically receive new information.

It is not a matter of the existence of teachings that elevate man in the religions, which number more than four thousand, but rather it is a matter of the ability to implement them and live by them, and this is what God uniquely does through faith in the Lord Jesus, not just mental and intellectual faith, but faith that believes in God’s work in Christ for the forgiveness of our sins. And granting us the righteousness and goodness of Christ, which results in living these very lofty teachings on the tongue of the Lord Jesus, such as loving enemies and others.