http://revach.net/article.php?id=3940

Section: Parshas HaShavua   Category: Medrash
Medrash Devarim: Is Pinchos Really Eliyahu HaNavi?
The Medrash Pliah says that whoever wants to know that Pinchos is Eliyahu can see it from "Eileh HaDevarim," (Parshas Devarim 1:1)  What does this mean?
 
The Shalal Rov brings from the Ishei Yisrael as follows.  The Yalkut Reuveni asks, "How can Eliyahu testify in Shamayim about the actions of people on earth if he is only one person and you need two Eidim for proper eidus?"  One answer given based on the Zohar is that after Pinchos killed Zimri, the neshamos of Nadav and Avihu entered him.  Therefore, he is two people and is qualified to testify.

This answer only works if Pinchos is Eliyahu, but what if he is not?  How would Eliyahu, who is only one person testify?  To this, the Medrash answers, that we must look in Sefer Devarim where the Torah says that you need two witnesses to testify.  Therefore, since Eliyahu testifies in Shamayim it must be that Pinchos is Eliyahu.