Right. But I think the idea is there are two groups of particles when both slits are open. When one is blocked a particle detector, they can't go through both slits.
With a detector in one of the slits, you will only see one group, not the unmeasured state with both groups.
That's not how the double-slit experiment works. Both slits remain open at all times. The detector only detects the passage of the particle through one of the slits.
This is the first explanation of a 'many worlds' theory of quantum mechanics that has ever made sense to me. The animations are excellent:
http://www.preposterousuniverse.com/blog/2014/12/16/guest-post-chip-sebens-on-the-many-interacting-worlds-approach-to-quantum-mechanics/