This Bible contradiction is from the Skeptic's Annotated Bible.
2 Samuel 21:19 View context
And there was again a battle in Gob with the Philistines, where Elhanan the son of Jaare-oregim, a Beth-lehemite, slew [the brother of] Goliath the Gittite, the staff of whose spear [was] like a weaver's beam.
1 Samuel 17:49-51 View context
And David put his hand in his bag, and took thence a stone, and slang [it], and smote the Philistine in his forehead, that the stone sunk into his forehead; and he fell upon his face to the earth.
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 of David.
Therefore David ran, and stood 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.
Lahmi, the brother of Goliath
1 Chronicles 20:5 View context
And there was war again with the Philistines; and Elhanan the son of Jair slew Lahmi the brother of Goliath the Gittite, whose spear staff [was] like a weaver's beam.