A common “difficulty” or contradiction cited in the Bible is the fact that the text tells us David “killed” Goliath twice. The passage is 1 Samuel 17:50-51. Here’s how it reads in the ESV.

So David prevailed over the Philistine with a sling and with a stone, and struck the Philistine and killed him. There was no sword in the hand of David. 51 Then David ran and stood over the Philistine and took his sword and drew it out of its sheath and killed him and cut off his head with it. When the Philistines saw that their champion was dead, they fled.

In my reading of Do Historical Matters Matter to Faith? Old Testament scholar Robert Chisholm offers the following explanation:

“The alleged ‘double killing’ of the Philistine in 17:50-51 can be explained reasonably when one takes a closer look at the Hebrew text. In verse 50 a hiphil form of מוּת, ‘die,’ is collocated with ‘he struck down,’ while in verse 51 a polel form of מוּת is used to describe how David killed the Philistine with the sword. The collocation of verbs in verse 50 has the nuance ‘dealt a mortal blow.’ The polel of מוּת (v. 51) is used in eight other passages in the Old Testament. In three poetic texts, it appears to mean, simply, ‘kill, put to death’ (Pss. 34:21; 109:16; Jer. 20:17). But in narrative (all in Judges-Samuel) it appears to have a specialized shade of meaning, referring to finishing off someone who is already mortally wounded (Judg. 9:54; 1 Sam. 14:13; 2 Sam. 1:9-10, 16). Abimelech’s statement (Judg. 9:54) is particularly instructive—he asked the armor bearer to kill him (polel) because otherwise people would say that a woman killed him (the verb is הָרַג, ‘kill’). So who killed Abimelech? Two answers are possible and both are correct—the woman (she delivered a mortal blow that made death certain) and the armor bearer (he delivered the death blow in the technical sense = polel). How did David kill the Philistine? Again two answers are possible and both are correct—with a sling stone (David delivered a mortal blow with the sling that made death certain) and with the Philistine’s sword, which he used to deliver the deathblow in a technical sense (= polel).” (p. 195)

[He refers to a PhD dissertation done by Joe Arthur, “Giving David His Due” (Dallas Theological Seminary, 2005), 83-85.]

This solution by Chisholm is reflected in a couple of contemporary translations. The Common English Bible for example reads: (See also the Message and God’s Word Translation)

And that’s how David triumphed over the Philistine with just a sling and a stone, striking the Philistine down and killing him—and David didn’t even have a sword! 51 Then David ran and stood over the Philistine. He grabbed the Philistine’s sword, drew it from its sheath, and finished him off. Then David cut off the Philistine’s head with the sword.

Chisholm offers a second possible solution but I won’t relate that here. This is proving to be a very interesting read. I recommend it highly.