The subject here is 1 Samuel 17-18, the account of David and Goliath. Critics propose that there were errors in the account which were best explained by there being at one time "2 or more different versions of the story" -- one of which, a shorter version, has been preserved in the LXX (which actually does not end up resolving all the alleged errors, but that is beside the point).
Such critical claims were composed long before there was any proper understanding of the ancient process of oral transmission and storytelling. Let us see how this resolves seven "inconsistencies" reputedly found in the story:
In 1 Samuel 17:55-58, when the young David approaches Goliath, King Saul asks his general, Abner, to identify the boy - asking who his father is. Abner says he doesn't know him. After David slays Goliath, David is brought to Saul and introduced to him. Saul asks who David is. Why do Saul and Abner fail to recognise David? and why is it necessary to introduce David a second time to Saul? Different stories!
No -- Saul does not ask who David is; he asks who his father is. Why?
Earlier in 1 Samuel, there is a specific reason to make a note reminding us about Jesse and that David was the 8th of the sons. Note that only the three oldest sons are named, and that they followed Saul. Since Saul's three oldest sons served also (cf. 31:6), it is likely that there was some sort of "draft" that requested that fathers send their three oldest sons for the sake of the war.
Thus David is being shuttled in and out of his position at court because he is needed at home to help replace the three eldest sons who are at war. That, of course, is the point in mentioning that Jesse is a bit on the oldish side.
Now as to the "father" verses":
As Saul watched David going out to meet the Philistine, he said to Abner, commander of the army, "Abner, whose son is that young man?" Abner replied, "As surely as you live, O king, I don't know." The king said, "Find out whose son this young man is." As soon as David returned from killing the Philistine, Abner took him and brought him before Saul, with David still holding the Philistine's head. "Whose son are you, young man?" Saul asked him. David said, "I am the son of your servant Jesse of Bethlehem."
Again, the question is clearly, who is David's father. Note also that David was only one of Saul's armor-bearers (16:21). We do not know how many of these there were under Saul, but if there were more than a few, say five or six, and if David had been away from court for a while, then it is certainly not surprising that in the heat of a pressed battle situation in which the fate of the nation is at stake, Saul happened to forget whose son was whose.
However, we would add that the question was just as well rhetorically designed for the sake of rounding off the story the same way it began in 17:12 --- something that makes perfect sense in an oral culture.
Now for number two:
In 18:17-19, which is missing from the LXX, [Saul] offers Merab. An explanation for the offering of two daughters has been added to the Biblical account, to reconcile the different names in each original version (v19 & 21b). This results in an odd and repetitious final combination.
This is cited as an "inconsistency" but there is no internal inconsistency here at all. What is the problem? There is none, and there is no "odd and repetitious final combination."
One instance, in which all scholars agree there is a mistake in the Biblical Version, concerns Saul's attempt to kill David. As it appears in 1 Samuel 18:10-11, the account has been misplaced by the biblical editor.
"All scholars" say no such thing, and there is no reason to think that 18:10-11 is misplaced. The LXX is cited as more logical because it gives Saul allegedly logical stages of consideration, but there is no need for this -- Saul was in a prophetic ecstasy when he did this (18:10) and his actions against David there were not germane to his heightening suspicion.
In 17:12, David and his father Jesse are again introduced to the reader.
Again, as in they were already introduced in Ch. 16. It is answered by noting that 1 Samuel 17:12 begins a new unit that was undoubtedly originally an independent unit of oral tradition. It was told without the previous material having been told, so that introducing the main characters anew is not the least bit extraordinary -- it is only when the introductory formula was included in the stage when the material was compiled in writing that it took an appearance that looked strange to our eyes.
In 17:12-31 and 55-58, [David] is an unknown shepherd-boy who happens to be visiting his bothers [sic] when Goliath challenges the Israelites to a duel.
Incorrect, as noted above.
According to 17:25-27, David hears that whoever defeats Goliath will be given King Saul's daughter in marriage as a reward. When David kills Goliath, Saul gives his daughter to him (18:17-19). But in 18:20ff, Saul looks for pretexts to convince David to marry his daughter, and David insists he is unworthy. The decision to give David Saul's daughter to marry is made well after David slew Goliath.
The text says nothing about the decision being made after David did the deed; it indicates that Saul waited to fulfill the promise -- and why not? There was a war going on. The Philistines were on the run, but far from finished, and they were still a threat to Israel.
Since marriage meant that a man became unavailable for a year (Deut. 24:5) of course Saul's promise to whomever would be assumed to be delayed until the fighting was over -- why would Saul want to lose a man, whoever it was, who would put Goliath in the ground? (That would be a good public reason of course; privately Saul also would want to get rid of David.)
How did David kill Goliath?...According to 17:50, David knocked over Goliath with a stone, killing him without a sword in his hand (17:50). But, in 17:51, David killed him with his sword. He killed him again!
There are a couple of points here. First, "killed" (muwth) often means "killed" but also means "destroyed". Some have also suggested that 17:50 is a scribal gloss intended to correct the impression that David was reliant on his armament, not God, for his victory.
I would suggest rather that a chiastic structure is at work to enable memorization of the key portion of the story:
So David prevailed over the Philistine with a sling and with a stone, and smote the Philistine, and slew him; but there was no sword in the hand (Hebrew: yad) of David. Therefore David ran, and stood (Hebrew: 'amad) upon the Philistine, and took his sword, and drew it out of the sheath thereof, and slew him, and cut off his head therewith. And when the Philistines saw their champion was dead, they fled.
Note that this answer is not negated by saying things like, "Either David killed Goliath with a sword or with a stone. He didn't kill him with both." The chaism is a solution in which artistic license resulting in intentional contradiction and/or error enters the picture, so that the semantic contract changes, and charges of "error" and "contradiction" become irrelevant. It would be like claiming that a work of Dali is in "error" when it depicts watches as melting, or saying that a Van Gogh "contradicts" a Picasso.