How can reincarnated souls be summoned using Edo Tensei? Quoting wikipedia,


  Reincarnation is the philosophical or religious concept that an aspect of a living being starts a new life in a different physical body or form after each biological death. It is also called rebirth or transmigration, and is a part of the Saṃsāra doctrine of cyclic existence.


Then, since Ashura has reincarnated into Naruto, and Indra into Sasuke, how can Uchiha Madara and Senju Hashirama (their previous reincarnation) be summoned using Edo Tensei?
As I've mentioned in the comments, there seems to be several fallacies in the base assumptions taken in the question. However, to answer the questions just based on the information in the Narutoverse, we've to see what exactly do Resurrection and Reincarnation means.

The translation of the Resurrection and Reincarnation techniques is quite similar in usage.


  The Summoning: Impure World Reincarnation binds the soul of a deceased person to a living vessel, restoring them as they were when they were alive in order to do their summoner's bidding. Naruto Pedia: Edo Tensei


Edo Tensei is a summoning technique that summons a "soul" residing in the pure land. Thus the so called "Resurrection" is a reincarnation of a soul binded to the impure land through its DNA marker and a sacrifice.


  Reincarnation is a process through which the chakra and will of a deceased individual are reborn in a new living vessel, referred to as a "reincarnate". Narutopedia:Reincarnation


Hence though the souls, i.e. the actual individuals would go to the pure land in the Narutoverse, their Chakras and Will would be "inherited"  by individuals in the next generation. These individuals would hence be their "reincarnate". In a single generation their can only be one Reincarante of an individual.

It is important to note, through the Summoning: Impure World Reincarnation technique, both "reincarnation" and "resurrection" can be found; the sacrificed body is reincarnated into the body of the summoned shinobi, while the reincarnated shinobi is resurrected from the dead.

Tl;Dr In the Narutoverse, both techniques are independent of each other. Through Edo Tensei souls residing in the pure land can be summoned, irrespective of the fact that their reincarnate exists in the current world.