Question

Si dos cuerpos sólidos tienen la misma densidad, están formados por la
misma sustancia

Answer 1

Pure materials ,they can not have same densities. (Assuming they are not isotopes of each other.) But mixtures, compounds or different phases can have same densities.

Answer 2

Answer:

I think you want to say that

If two solid bodies have the same density, they are made of the same substance

No it is not compulsary that they are made of same substance.