Salam Alaikum,
The point is not difficult to understand. In verse 7:38, both will get double punishment; one for following the wrong path and the other for misleading others, those who came after them. This does not contradict verse 6:160. The double nature of sin is confirmed in many other verses;
“They not only go astray themselves but also induce others to follow the wrong path. Therefore, on the Day of Reckoning, they will bear, not only the full weight of their own burden, but also part of the burden of those ignorant ones whom they misled. How unbearable will be the burden they will have to bear!” (16:25)
But they shall, surely, bear their own burdens, and other burdens along with their own burdens.(29:13)
And they say : Our Rabb! Oh ! we obeyed our chiefs and our great men and they misled us from the way. Our Rabb! Oh ! give them double torment and curse them with a mighty curse (33: 67-68)
Similarly, the one who commits a crime and throws it on another, he commits a double crime: "And if any one commits a mistake or a crime himself and then foists it on to the innocent, he carries (on himself) (both) the burden of a calumny and a flagrant sin."(4:112)