Identical twins are two beings developing from the same fertilized egg. They share the same genetic material and the same amniotic sac.
Original thoughts were that pups can be fraternal twins or triplets meaning they have the same sire but are not identical. Half siblings can have the same mother but different fathers. Often what are believed to be twins are simply placentas that grew together. Also pups can have identical markings but are not necessarily identical twins.
Taking all that into account, there have been verified reports of identical twin pups being born. But the only true test that can prove that pups are identical twins is a DNA test.