According to Jeremiah, God is saying that a place called Tophet will become very sad and full of bad things because there will be too many people who are hurt. There won't be enough space to bury everyone, so it will be called the valley of slaughter instead.
Jeremiah 7:32: Therefore, behold, the days come, saith the LORD, that it shall no more be called Tophet, nor the valley of the son of Hinnom, but the valley of slaughter: for they shall bury in Tophet, till there be no place.
This verse states a similar outcome for the Valley of Hinnom, yet reiterates the prophecy rather than contradict, showing fulfillment rather than conflicting prophecy.
Jeremiah 19:6: Therefore, behold, the days come, saith the LORD, that this place shall no more be called Tophet, nor The valley of the son of Hinnom, but The valley of slaughter.