Question

why all the co bond distance in Na2CO3 are equal

Answer 1

The bond lengths are the same because of this continuous position shift in the double bond. Lets be clear, they do change a bit but the shift in the position of the double bond is very quick that they practically seem constant and seem to be everywhere. Because of this, we simply assume that the pi-bonds therefore the pi-orbitals of all atoms have been merged together, out of sheer convenience.