After that, Aaron returned and reincarnated into Uriah the Kohen of Kiryat Ye'arim, who was killed by King Yehoyakim. Through this, he was forgiven for the death for which he was culpable as a result of the (Golden) Calf. However, Nadab and Abihu also reincarnated with him into Uriah the Kohen as an actual gilgul, since they died because of him, as we said on the verse, "God became very angry with Aaron to destroy him." (Deut. 9:20)

...Nadab and Abihu also reincarnated with him into Uriah the Kohen...

Therefore, ARYH/Uri(y)ah was named after AHRN/A(h)aron - their names are similar and both were priests. In addition, even though he was rectified when Uriah was killed, since he caused Nadab and Abihu his sons to reincarnate and be killed because of him, therefore he needed to reincarnate a second time into Zechariah the prophet and kohen, the colleague of Haggai and Malachi. Nadab and Abihu reincarnated with him as a real gilgul.

Thus Uriah the Kohen, who came to rectify the sin of Aaron, was called kohen, to allude to Aaron the first kohen. However, Zechariah came for the purpose of Nadab and Abihu, and therefore was not called kohen even though he was one (and they were also kohanim). As it says, "I appointed trustworthy witnesses for myself: Uriah the Kohen and Zechariyahu the son of Yeberechiah." (Isaiah 8:2)

There is also another hint, for with respect to Uriah it does not mention "son of", though it does with respect to Zechariah/Zechariyahu. This is to allude to the fact that Uriah was the father and Zechariyahu was the son - that is that Nadab and Abihu his sons were the principle souls in Zechariah.

...each of them had reincarnations of two "witnesses"...

Therefore are they called "trustworthy witnesses" - because the two of them are one (the same) soul, as mentioned, and each of them had reincarnations of two "witnesses" - an aspect of Aaron and an aspect of his two sons. Hence, Uriah also prophesied about chastisements since he still needed to be killed. However, Zechariah prophesied about times of comfort and the building of the Second Temple, as found in his book.

A witness comes to clarify the truth in places where it is hidden. (Likutei Moharan) These tzadikim came to fix the root of Aaron and prove its absolute goodness from amidst its falls. They thereby testify to the Divine unity, which rests on and manifests through tzadikim. This is one idea behind "tzadikaya anpei Shechinta/the righteous are (as if) the face of the Divine Presence". (Zohar)

Each prophet expresses his vision in a different manner - according to his soul root. (Chatam Sofer 15:16) Also, the nature of the prophesy given to him to tell relates to his merit and his own tikun in the world. Isaiah accepted upon himself the yoke of Heaven with joy and so merited to foretell times of happiness. (Yalkut Shimoni Isaiah 6) Uriah had to work on the din side, which involves suffering - therefore he had to give over messages of reproof. After this tikun, the door was opened for Zechariah to do it on the chesed side.

[Translation and commentary by Perets Auerbach.]