What is meant by "and the earth has been stretched forth" in Surat Ar-Ra'd and "and the earth is stretched out" in Surat Al-Inshiqaq, and is there a difference between the two instances of stretching forth?

The "madd" (extension) in His, the Almighty's, saying: (And it is He who extended the earth) means spreading out, i.e., He made it vast, extending in length and width.

As for the "madd" (extension) in His, the Almighty's, saying: (And when the earth is extended), it means an increase in its spaciousness and its being spread out, and the removal of mountains and everything on it so that it becomes extended like a hide, to accommodate all creation for accountability. This "madd" differs from the first "madd," and it signals the demise of the world.