Prevent particles at the same place? I try a simple particle system, with this group on the layer 2 :

The result, placing it at Vertices places on a grid, using this group, is like :

The problem is that sometimes 2 or 3 objects are at the same time on the same place... I didn't found anywhere how to disallow that, I would like to have just 1 object at random places (less or equal to the number of vertices)
Thank you !
blend file
I think this way may meet your needs:



See key settings marked in the image above, very intuitive I suppose.