1 Samuel 17:4,7,50 -- David was victorious and slew Goliath
2 Samuel 21:19 -- Elhanan slew Goliath
This apparent contradiction is self made for it cannot be a real contradiction because of various points. The record in 1 Samuel 17 is about the victory of the young David over the giant from Gat, Goliath, when the Philistines had come up against Israel during the time when Saul was still king over Israel. The record in 2 Samuel 21 however describes the deeds of David's mighty men during various wars against the Philistines during the time when David was king over Israel. Thus, these two recorded victories over a "Goliath" must be two distinct events.
Another point is still to be noted about the "Goliath" for which one must take into consideration a parallel record to 2 Samuel 21:19. In 1 Chronicles 20:5 this event is spoken of and there it is mentioned: "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." This indicates that the verse in 2 Samuel 21:19 must have an ellipsis (the reference to "Lachmi, the brother of" Goliath) by which the emphasiswhich actually has been correctly supplied in the KJV, and by which the emphasis is put on Elhanan who had been victorious over a giant, a "Goliath", among the Philistines. Of course, this could not have been the same Goliath, who was killed by David before, it was instead Lachmi, the brother of that Goliath.