Presently there are a number of hypotheses attempting to disentangle the sources and composition of the OT. The 19th century approaches (Wellhausen et al.) opened the world's eyes to the reality that the Torah and histories are the work of a number of contributors with particular agendas and vocabulary. Unsurprisingly there have been refinements, or at least recognition of difficulties, with early proposals. A point in dispute is the role of redactors, who, when, why, how many.
There are often legitimate reasons for disagreement, such is the nature of textual criticism. Often the suspected redaction (or even a source) involves a single word or phrase that interrupts the flow, is anachronistic, or contradicts and unfortunately, without supporting evidence, what may be recognized as a redaction by all, is difficult to be certain as to the who's and when's.
Supporting evidence is difficult given the lateness of our extant documents. Stylistic, idiomatic and theological markers may not provide enough to be dogmatic. Therefore, it's not a failure to admit may need to be willing to be less than certain about details about the bumpy road the OT took to take the form we have today.
However, what follows is an interesting example of a redaction that has a number of good lines of evidence that converge. This is mostly because the redaction/interpolation was pretty late in history.
In the 1 Kings 6 description of the building of the Temple:
He constructed the House and completed it. He paneled the House with beams and planks of cedar. 6:10 He built the storied structure against the entire House—each story 5 cubits high, so that it encased the House with timbers of cedar.
6:11 Then the word of YHWH came to Solomon, 6:12 “With regard to this House you are building—if you follow My laws and observe My rules and faithfully keep My commandments, I will fulfill for you the promise that I gave to your father David: 6:13 I will abide among the children of Israel, and I will never forsake My people Israel.”[13]
6:14 Solomon constructed the House and completed it. 6:15 He paneled the walls of the House on the inside with planks of cedar.
The block of text (11-13 and the repetition in vs 14) are regarded as an interpolation. The vss11-13 section interrupts the narrative flow with a new appearance of Yahweh to Solomon. This created the necessity to repeat vs 9 in vs 14 to bring the readers' mind back to the topic of completing the Temple. As we will see, not only does the narrative flow suggest an interpolation, but the larger context does also.
In 1Kings 3 Yahweh appears to Solomon at the Gibeon high place:
King Solomon went to Gibeon to offer a sacrifice because that was the most important high place. He offered a thousand burnt offerings on that altar. 5 While Solomon was at Gibeon, the Lord came to him at night in a dream. God said, “Solomon, ask me what you want me to give you.”
and yet later in chapter 9 we read:
YHWH appeared to Solomon a second time, as He had appeared to him at Gibeon. 9:3 YHWH said to him, “I have heard the prayer and the supplication which you have offered to Me…”
Clearly the original narrative did not include the appearance in chapter 6 that we are discussing.
What's really unusual in this case is we also have documentary evidence for this being a late (proto-Masoretic) redaction through the reading in the LXX (msB). Note the verses with the appearance to Solomon are not found. Α΄ Βασιλέων (1 Kings) 6 (LXX) - καὶ ἐγενήθη ἐν τῷ τεσσαρακοστῷ (blueletterbible.org)
Regarding why the redactor (likely a 3rd century BC scribe of a school that ideologically identified with the Priestly source) felt the need to insert this new material, it seems consistent with a number of other similar redactions that went to pains to insist the Davidic covenant and Temple were conditional. Simply said, it supplied textual support for their explanation offered for the loss of the Temple and independence. Unfaithfulness.
In my understanding, this redactor's outlook quickly became a minority view, it might already have been when this was done. (e.g. Deutero-Isaiah 2 centuries before argued differently)