A billion people are living in slums, not the same billion people, but there is some overlap.
2017年6月六級真題（第二套）聽力 Section C
柯林斯高階英漢雙解學習詞典釋義
V-RECIP-ERG
與…互搭；與…復疊；（使）相疊
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 .