If the revelation in section 16 is virtually identical to Doctrine and Covenants 15, why include both in the record? John A. Widtsoe noted,

The Doctrine and Covenants is a compilation of the revelations received by Joseph Smith to individuals for the guidance of the Church. From the first years of the work the Prophet kept every scrap of paper pertaining to the progress of the work. In fact, this care of things that must have seemed trivial is one of the evidences of the sincerity of the man. For example, when John and Peter Whitmer asked for help, he received for each of them a revelation, substantially the same. . . . This simple revelation is directed to the individual and at first sight has no permanent value for the Church. Yet as a revelation from God it was preserved and published. An insincere man could have eliminated this and other similar revelations as of little consequence. Not so with Joseph. The Lord had spoken. The words were part of the building of the kingdom of God, and the same advice would be useful to many men then and now (Joseph Smith: Seeker After Truth, Prophet of God, 1951, 251–52).

Peter Whitmer Jr. later served as one of the eight witnesses of the Book of Mormon. He also served as one of the missionaries who accompanied Oliver Cowdery on his mission to the Lamanites [American Indians] (D&C 30:5). At a conference held in Hiram, Ohio, in October 1831 Peter said, “My beloved brethren, ever since I had had an acquaintance with the writings of God I have eternity with perfect confidence” (Minute Book 2, 12, JSP). He later served as a leader of the Church in Missouri, enduring the expulsion of the Saints from Jackson County. In 1836 he was appointed and set apart to the Missouri High Council (replacing his brother, Christian, who had passed away). Peter himself died from tuberculosis on September 22, 1836, shortly after this appointment. He never denied his testimony of the Book of Mormon (Lyndon Cook, Revelations of the Prophet Joseph Smith, 1985, 26–27).

While most of the Whitmer family left the Church in 1838, Peter Jr. and Christian Whitmer never did. Shortly after their deaths, Oliver Cowdery (a Whitmer brother-in-law) wrote the following tribute to Peter Jr. and Christian:

Among those who have gone home to rest, we mention the names of our two brothers-in-law, Christian and Peter Whitmer, jr. the former died on the 27th of November 1835, and the other the 22nd of September last, in Clay county, Missouri. By many in this church, our brothers were personally known: they were the first to embrace the new covenant, on hearing it, and during a constant scene of persecution and perplexity, to their last moments, maintained its truth—they were both included in the list of the eight witnesses in the book of Mormon, and though they have departed, it is with great satisfaction that we reflect, that they proclaimed to their last moments, the certainty of their former testimony: The testament is in force after the death of the testator. May all who read remember the fact, that the Lord has given men a witness of himself in the last days, and that they, have faithfully declared it till called away (Messenger and Advocate 3 no. 3, 426).