A billion people are living in slums, not the same billion people, but there is some overlap.
2017年6月六级真题(第二套)听力 Section C
柯林斯高阶英汉双解学习词典释义
V-RECIP-ERG
与…互搭;与…复叠;(使)相叠
If one thing overlaps another, or if you overlap them, a part of the first thing occupies the same area as a part of the other thing. You can also say that two things overlap .